Hey, welcome to my life!! Except that my ds, who turned 4 in June, is hitting us too now. Sigh. I really truly believe this is a stage, and I have read about it being a stage in the "Your 4-yo, Wild and Wonderful" book, have you read those? They are somewhat outdated (ignore the advice) child development books from the 60's or 70's, and they just have a bunch of basic info on what's going on with your child at the different ages.
Anyway, 4yo hitting (and kicking, and spitting, oh joy) is supposed to be way normal. They are ahead of themselves physically and mentally now, and still waiting for the emotional maturity to catch up.
I think the trick to is is how you respond. We have had varying success, and I am trying to figure out a way that feels right to me. I've ranged from ignoring it, but don't want to let him feel that it's ok to hit, especially not women, all the way to whacking him back, unfortunately, something I'm not proud of and didn't plan, but I was hit a lot in childhood, and my immediate reaction is to defend myself. I think there is a better way, somewhere between the two extremes, I just haven't found it yet.
Anyway, I've been working on increasing our connection, I just bought "Playful Parenting" and have been playing with him more, which helps, and have been trying new ways to avoid power struggles, which is how the hitting always seems to start in our house.
I hope some of that helps, just know you are not alone!
People whose kids have different personalities just don't understand, they are like, oh it's so shocking, how can your kid hit you, oh he must be messed up etc etc, but really, I think some kids just have a harder time channelling those negative emotions, and are going to need more guidance for a longer time. At least mine will...