Toasty, I really sympathise with you. My oldest son went through a similar phase. Reading your second posting it sounds like you know all about the benefits of coping calmly with his tantrums - it's just that reality doesn't always match ideals.
Please put lots of IMOs in front of anything else I now say, because I am only drawing on my own personal experience with my son.
Firstly I really wouldn't feel at all bad about giving him 'time out' or even, on the odd occasion, sending him to bed. Temporarily depriving him of attention when he's badly behaved is not the end of the world and it seems many childcare experts say it's a valid form of control. And it helps stop you losing control too.
Also, remind yourself that the worst is behind you. Your son is nearly four. So nappies, sleepless nights and pushchairs etc are fast on the way out. He talks and walks ok. He can make his needs known to you. He's beginning to know the rudiments of safety and appropriate behaviour, so knows that pavements not roads are for walking on etc. You no longer have to be three steps behind him in the playgroud or try to anticipate his every move.
Ok you have tantrums but if my son is anything to go on 4 is when I began to see light at the end of the tunnel. I think 4 is a lovely age - the beginnings of independence, so you begin to get some of your life back and yet, pre-school, they are still so innocent and unintentionally funny and you are still so important to them.
Despite this, I went through a bad patch with my son when he was about five. I was under a lot of strain: work, family, financial, decorating, selling house, moving, and getting pregnant and giving birth to my second. I had so little time for anyone or anything, and when I wasn't working or decorating, felt constantly tired. I simply didn't have much left of anything to give my son, although at the time I found it hard to admit this, as I arranged day trips and fun things to do. We did enjoy them, of course, and it wasn't all doom and gloom, but I was really only firing on one cylinder. So, when he misbehaved in any way, this was often the last straw and I would have great screaming matches with him.
What made it worse was that ds had a much calmer relationship with my husband, and was far better behaved with him. It was so noticeable that quite a few people commented on it to me. That and the fact that my son could be an absolute charmer around adults when I wasn't around meant it was easy for me to feel I was really not motherhood material!
Anyway the bad patch passed and the stress and strain etc got less. I suppose without realising it I seemed a slightly different mummy to my son. I certainly had more time to be with him. I also made a conscious decision to see him as a good boy who sometimes did bad things, as opposed to a bad boy who had to be persuaded to do good things. To do this I had to let some of his minor bad behaviour go unpunished for a while, so we could concentrate on the good bits, and followed the positive praise advice you see in books. I didn't stop losing it and shouting at him entirely, but I tried to create more nice times together to counterbalance it. No one's perfect are they? but at least you can then look back on a day with your son and remember some good times.
Anyway going back to tantrums at four years old. Have you tried telling your son you're going to phone daddy or his grandparents - or even pretend you're having a short conversation with them. I used to tell my son - daddy has just said 'go to your room'. When we were out I used to say a policeman will come and tell me off if I let you do that. Cheating I know, but it helped change a tantrum from a mum versus son thing into a good versus bad thing.
Also, if there are certain situations that are likely to produce bad behaviour or screaming matches try to eliminate them and follow your own instincts: I wish we had not given away our stroller so soon. Even when he was four, my son still wanted to be carried if he was tired. If only I had had a stroller to hand, it would have saved so many arguments when we were out. I took my cue in this from other mothers, not my son, and he knew it, and that made him even crosser.
Also I began to have a bedtime chat with him. There was no pressure on him to do anything because he was fed, washed and in bed. I didn't have to be the bossy mummy he'd had all evening. I didn't have to come across like a mummy at all, just someone really interested in him and his day. For me this has been a really good thing. I helped me to like him, as well as to love him to bits. Can I be outspoken and say that I don't think 'like' automatically follows love when you are a mother. You can't 'like' until you have a personality there to get to know and to make friends with. Babies don't have much of this. Toddlers increasingly do - and I suppose for me at least, this revelatin came as a shock. I just thought love alone would carry us through and then felt I'd failed because it didn't.
I am rambling now and lots of this may be off the subject, Toasty, but I send you my best wishes.