"I live inside my head a lot of the time and some days (like today) every noise feels physical."
I don't have any solutions, but just wanted to say that I really relate to this. Also the whole can't live with them/can't live without them thing during the tough times.
I think 4 is particularly tough - I remember ds1 (now 6) going through a horrendously stroppy period around 4 years old - I think they get a boost of hormones or something around this age, don't they? Sure seem like moody little teenagers some times, anyway.
I think mostly that when I feel like that, I just try and take it day by day, or if that doesn't work hour by hour... minute by minute if necessary. It's so easy just to drown in despair if you try to look ahead too far, and the reality is that they do change over time (and bring whole new challenges just as you got used to the old ones.. heh).
I do find though that I'm enjoying parenting a school age child, and am finding myself really looking forward to my 3 year old (who is 4 in May, heaven help me) starting school this September. Every few months the care becomes slighly less physical and draining and more mental/emotional which I find I enjoy more. I love going for a walk with my 6 year old and having in depth chats about how far away the moon is, or what it would be like to have a dinosaur as a pet... have fallen in love with him all over again now I can see his mind developing and started to share HIS inner world.
Hang on in there.