I have no advice but I can commiserate!
My DS turned 4 in August and has recently started acting very angry at home. Hitting, spitting, "i hate you...you are stupid", etc. He has always been a bit anxious (I am too) but he just seems so negative for a four year old. I have a difficult time taking him to the playground because I'm afraid what might happen. However, his teacher (he is in PreK for three hours a day five days a week) says he is very shy and quiet, doesn't stand up for himself.
I've called the pediatrician who told me to fill out a "Vanderbilt Questionnaire" but that deals with ADD/ADHD. Personally I think it is low self esteem.
I plan to follow through with whatever the pediatrician recommends so I will keep you posted if interested.
Here's the link to the Vanderbilt Questionnaire: