OP I know how you feel. I am struggling a lot with my 4 year old as well. I am finding it an incredibly challenging age.
I went through an absolute low point and ended up scanning millions of threads on here, downloading about six different parenting books on kindle, I even started a facebook support group! (I had been meaning to start a local one anyway as I'm moving away, but still!) I also found the "Has parenting affected your mental health" thread on here amazing and brave and freeing.
It's the constant demands for attention, and not just attention but ABSOLUTE, full attention and nothing less. It's the terrible jokes which come in a constant stream and you lose the will to live with your cheerful fake laugh after two but they go on for twenty. The inability to express a single sentence in less than 73 words. The toilet humour. The obsession with things that I just cannot bring myself to care about. (I curse the people who invent children's TV shows and fads which have absolutely zero adult entertainment value.) The whining, the constantly trying to assert themselves over things that are really not that important (and I know, it's important to him but sometimes, really, I don't want you to struggle to reach the lock in the door for 5 minutes, I just want to GET IN and sit down). That point of the day when you blow up because they've done something unreasonable which is entirely reasonable for a four year old because they've been teetering on the edge of (what feels like) goady behaviour all day and this is what tipped it. And then you feel bad. The trail of mess and destruction they leave in their wake, and by the time they finally succumb to sleep you're too exhausted to deal with it.
It is tough. I am seeing an improvement in the last few days so I'm going to share what has worked for me and hopefully it helps you.
I noticed that his behaviour is best and most reliable and we get on best in situations where the boundaries are very clear and definite and he knows what to expect - this is when we are walking somewhere (especially if he is scooting) and bedtime when we read stories. The first because I have always been utterly consistent due to safety and the second because I got so stressed it made it easier to stick to "If you sit nicely and listen I will read the story, if not I'm going downstairs." When there is a clear and logical consequence, I am able to apply it consistently and calmly and this works and he is so much more settled. So, I stopped trying to be all soft and touchy feely and emotionally connected ALL the time. I offer one opportunity to rectify by pointing out that what he's doing isn't a great idea because of X and offering an alternative and if he doesn't take it pretty much straight away, or starts showing an attitude about it, then I go to plan B which is to threaten the dreaded generic punishment. I jest because this was always my horror, that I would end up relying on one, but it prevents us from getting locked into a battle which I might win by force but ultimately lose because it's a stupid example to set him. So he gets banned from TV for anything from half an hour up to potentially infinity - it's never gone further than bedtime, so far. It's not scary, it's not a huge battle to enforce, and actually he doesn't sit around whining that he can't watch TV, he moans for about 5 seconds and then finds something else to do. It breaks the tension and it's proving to make a difference, in that he's less likely to pick a battle over every little thing because I'm refusing to engage in the big battles any more.
The second thing has been to introduce a currency which prevents him from whining about things he wants constantly. Some people start giving pocket money at this age, DS gets too frustrated because he doesn't understand money at all, so we go for stars linked to behaviour. I don't give stars for every little thing because I think that is counterproductive, but sometimes I set him a challenge (like, carry this bag home for me without whining) or sometimes I award one after a particularly challenging thing for him (like having his hair washed) and sometimes he asks, if I haven't recognised something and he thinks I should. I don't always say yes, but sometimes I do, and if I don't then I explain why. He has to get 5 stars to buy a magazine, or he can save up 10 to get a DS game. It stops the whining for a magazine constantly because I say "Not this time, you have to get 2 more stars" and he understands. Sometimes he is still argumentative and "I don't care about stupid stars, I don't want one anyway!" but, well, that's his loss. It takes 1-2 weeks to gain 5 stars depending on how co-operative he is being in general. If it's sweets then one friend suggested to me to have a set "sweetie day" and only ever buy sweets on that day - helpful when they are starting to get the idea of days of the week.
DP isn't around for me at the moment, but he will be soon and I am looking forward to having his support again especially at bedtime when I typically get the most stressed out. If your DH is around then enlist his help as much as possible! Just knowing that you can step out and say "I need you to take over because I'm about to lose it" really helps.
Then the other things - slowing down and lowering your expectations. One of my friends gave me a great tip which is setting yourself up to win and having goals for the day which are really low so that it's easy for you both to achieve them and then you feel good. Remember she's only four and although she's probably really articulate and talking all the time, she doesn't even know what tomorrow is. She is still tiny really and I think it's easy for us to forget that when they are starting to look and act much older. Go back to some of your two year old techniques - pre-warning and explanation of what exactly is going to happen and your expectations of her before you do it is great. When she does try to communicate something in a way which comes across as rude or annoying, take a mental step back and count to 10. Let her finish the incredibly long winded sentence before you put your input in, let her get the whining out and then correct her in the way she could express this without whining. Try to sympathise with/acknowledge the way she is feeling even if the way she expresses it makes you feel stabby, or you feel she is being ungrateful. (How to talk is good on this too)
Today for example we were walking home from the supermarket and I had asked DS to carry ONE bag, which I'd put all the light things in, crisps, his new pyjamas and some salad leaves in a bag. I offered him a star if he could carry it home without whining. He said he didn't want a star anyway. I refused to move until he picked up the bag. He asked me to take one thing out, so I took out the salad leaves (which weighed nothing, so no problem to me). I offered to let him carry another bag instead. He agreed his bag was the best one. We barely got across the car park before he started whining that it was too heavy and his arms hurt and his legs hurt and he was really tired. I managed to take the mental step back, despite the fact my own back and arms were starting to ache from the 2 carrier bags plus full backpack, instead of snapping at him to stop whining and my bags were much heavier, I said, yes, my arms are getting tired too. Maybe we can have a rest halfway home and sit down for five minutes? So we crossed the road, I identified a point at which we could stop and he ran off, all full of renewed enthusiasm. We stopped for a break (I probably needed one too to be fair!) and I did not hear one more whine or moan from him the rest of the way home. Much better! And he felt helpful for carrying his one bag. I even told him that when he was a bit older he could carry more bags if he wanted to help some more and for once he seemed pleased instead of heinously offended!
I've also started trying to keep track of how he is handling a situation (e.g. being out) and taking him home if he's starting to get ratty rather than hanging on but getting annoyed with him. Finding acceptable activities to do at home helps too - I have found lego is a good side by side one as in he will ask me for help finding the pieces but I sort of hover and say "Have a really good look and then ask me for help if you're stuck" and I can do other stuff while he does this - similar with painting and drawing. When he tries to take over stuff I am doing with him, try to remember that what's important to them is that you are there with them. Just suck it up for 15 minutes - set a timer if you want to. There are times to teach them about co-operation and kindness when playing and there are times when it's just about being present even if you're mentally making a shopping list or something. Definitely take the pressure right off over things like sleeping and eating. If she senses that you care whether she does it or not it becomes a thing to latch on to and fight for. Stop caring. Provide the opportunity, and give her the responsibility. She won't starve, and it won't harm her to play quietly in her room until she is tired - you can have your evening without her there. I quite often say I'm going to eat my dinner now or watch a scary grown up DVD so I can't come up for a bit but I will come and check on him when I'm finished and if he feels tired to just lie in bed and wait for me - normally when I go up he's gone to sleep. If not then it just takes a few minutes of sitting on the end of his bed at that stage.
I think when your youngest gets a bit older it will get easier too, because they are so reliant on playmates at this age and if there are no other children aged 2/3+ around, then that playmate is you. That's the part I find really hard. (Playdates help.) From what I remember 8 month olds are pretty awful sleep wise too, which won't be helping!
As any other painful stage, it will pass, you just have to love them through it. The good parts: Watching them sleep when they have caused havoc all day. Watching them learn new things, amazing things. The brief moments when you reconnect and you remember what the relationship between you was like when it was constant and you see a glimpse of what it will be like when they come back to you again. I really think it's part of them asserting their own personalities and saying "Hey, I am my own person, not yours" but they still so desperately need you. You can be there for them even if you don't enjoy every minute of it. It's okay not to enjoy every minute of it!
I wish you luck and I hope that the phase passes soon and you can look back on it and realise that it was hard but you came through it intact and everything in the long run is okay. BTW, the being ultra-consistent thing is what works with DS but might not work with your DD, but if you look at the times she is most settled or most responsive with you, then you may be able to work out from that what the best strategy is.