I know exactly how your nanny feels Chipkid... I'm in the same situation myself at the moment - though the child concerned is older (age 7). Having been with the family for 4 months, you would have thought that a 7 year old got used to it... alas that isn't the case.
While I adore the younger children (1yr and 2yr), after-school issues with oldest child does impact on how much I enjoy the job. School holidays were fine, well as good as could be expected, but once back at school, it all kicked off again when he came home. I do get the feeling that there are things going on at school which he does not tell anyone about... but as I've said to him, if he doesn't tell anyone, noone can help him.
It's the aggression that's worse than anything. I think he just build up so much agression at school that once home, it all releases and I, being the one he doesn't want, gets the lashback. Given the choice, he wants mummy to stay at home.
Yesterday, he refused to eat tea (as per usual) and threw it at me, and then proceeded to jump and down on the dining room table! He got manhandled to his room - can't put up with behaviour like that, the younger children start to copy all too easily. He was then like a bull in a china shop, and didn't even stop when his dad got home.
I did think it would get better as time went on, but now 4 months down the line, it's just the same. Have started looking at other jobs and may get offered one from a family I met on Monday shortly. Will the oldest child's behaviur affect my decision to leave... yes. While leaving the younger two will be hard, my personal safety is important and if his aggressive behaviour can not be controlled, it is safer for me not to be there.
The other job will also pay more money - hmm, another factor to consider!
Sorry to rabble on, but as it's similar to your situation Chipkid, thought you'd like to know the view from the nanny in that situation. Talk with your nanny about it, agree on what tactics can be used to encourage better behaviour from your 4-year-old and if necessary impose rules for your son with consequences for breaking them, so that bad behaviour is strongly discouraged. Consistancy is vital, so same rules when you are at home and when nanny is caring for him.
Hope it does get better for you. Hope your nanny doesn't leave... but don't rule out the possibility - do talk with your nanny at regular times to make sure they are still happy to work for you... and offer incentives to make them happier if it will help!