You'll know this already, of course...4 is an age for power and control, your ds is probably acting it out on those near and dear to him.
I have a 4y/o ds at home, no others, but when I am looking after other little ones and there is the classic power debate, I try to turn it on them: ie, this morning:
Dropping off my ds and his friend at our childcare co-op, they see another child and start to make really un-kind comments about how icky so and so is here (well I couldn't believe that *my* precious one was saying these things about another child!! OMG! yikes!)
First I said that those are very un-kind words, and looking at his friend (who I must say is quite influenced by an older sibling), said that we never speak that way about other people and how would you feel if someone had said that about you?
I said it would hurt *my* feelings to know that someone would say those things about either of them, and please do not say such things about another person.
Well, ds in his rather consistent style of wanting to be the polar opposite of me (at 4!), said that it wouldn't hurt *his* feelings if someome were to say that about him.
I just looked at him with that mommy stare I have unconsciously started doing, and said, it is not ok to talk rudely about other peplke. Please do not do it again.
'Oh, all right' -heavy sigh from ds-
This may not be too helpful, huh?
Keep on reminding ds that he can make his own choices, as can dd. Was he having yogurt for breakfast, too? Maybe he wanted her to be 'just like him'?
These little ones and all thier thinking, oh my. Good luck. ;-)