I find 4 a very wearing age. I have three children 16, 6, 3 and it's been tough with all of them. 3yo is 4 in August and not quite in the frustrating stage yet but getting there.
I think for me it's two things - my eldest two both have ADHD, and it makes them very intense, but 4 is too young for it to have been identified in either of them so I was just sort of stuck windmilling against this little stubborn troll/emotional tornado. Better with DS2 (as in, my second son) because I had some idea plus had been there before, but still trying.
But also I think I am very well suited to parenting the toddler years (roughly 18 months - 3.5) and the 4-7 ish age is a very different stage and I struggle with it.
I don't know if everyone would like him, but I've been finding the reels by iam.mrluke helpful in rekindling some appreciation for this age, his videos where he play acts the ~5yos in his class are very sweet and I find that I can sort of adopt his "voice" in responding to my 6yo and it helps avoid some of my frustrated/irritated responses, which just escalate things rather than help.
4 is different because - they feel so much older, and at times they can be very sensible and grown up so you can get this sense that they "should know" things by now e.g. rules or expectations, but they are still very little. When I was struggling with DS1 many years ago someone on MN said "4yos are still babies really" and I've never forgotten it. It seemed incredible to call them babies when I compared him to my friends' younger children, and I think it's even more stark when you have two close together in age so you can see a literal baby as a comparison. But compared with much older children, they really are at a very young stage of development still.
Connection is only part of the story - I think this can be a harmful narrative because some resources suggest if you have enough of a connection, then they will be perfectly behaved at all times so if you are finding them difficult, it's your fault for not being connected enough - which is unhelpful. Obviously if they don't feel connected and listened to, that's not good, but they can also just act like butts for absolutely no good reason! That's four!
What I do find helpful is to dial back expectations a little bit and be proactive with physical needs - regular small meals and snacks, keep reminding them to go to the toilet, have a "try for a poo for 5 mins" planned into your bedtime routine. Not too much screen time, and try to be mindful/observant of what effect different kinds of screen time have - certain youtube channels drive my kids absolutely hyper and have been banned. In fact, in my house it's better if they only watch iPlayer or something like Netflix (and again, go through and ban annoying programmes). Time limits as well, especially if they are playing computer games. You'll probably need to experiment if this is an issue for you.
Also predictable structure throughout the day, use a visual schedule/timetable if you like, this seems to work well. Breaking the day into sort of "buckets" helps me with this, and trying to get everyone outside for some fresh air and exercise. If I really, really can't do this then something physical inside helps a lot. DH is king of dance parties. I tend to set up obstacle courses or put a single duvet down on the floor and roll them up in it then pretend they are a "sausage roll" with much pretend chopping, seasoning and eating which they find hilarious and can help a lot.
I think the thing with 4 is that you have to be much more explicitly in command, often against what they want to do, and I struggle with that, whereas toddlers will go with the flow a lot more, and older children can have more autonomy without it being a disaster. Does that ring true to you?
Best book I have read at this age is "When Your Kids Push Your Buttons" by Bonnie Harris, which is only 99p on kindle.
Other things which have been useful:
Janet Lansbury - blog/podcast, on holding boundaries without threat/punishment and some stuff about emotions and being a confident leader.
Occuplaytional Therapist - big mix of sensory stuff and unconventional parenting takes, parent emotional regulation, some developmental stuff.
ABCs of Everyday Parenting course (free, on Coursera) - positive reinforcement, breaking behaviour expectations down into smaller steps to make them more achievable.
TheOTButterfly - sensory info, which can help keep them in a better mood
Big Baffling Behaviours by Robyn Gobbel - explains regulation using animal metaphors. Book is better than podcast IMO. Helpful for "WTF do I do in the moment when they are crazy??"
Dr. Becky - I find her mixed, but I like her approach of giving 1 thing to do in the moment and 1 thing to do outside of the moment.
theteachermomma - She is a US Kindergarten teacher, and has good insights about this age (~4-7) and tends to pick up on common struggles parents make with moving from parenting more appropriate to younger children into this stage.