You might give the book "Parenting with Love and Logic" a try. I've found that very effective with my children and my students.
You might also try something my MIL used: goody tickets. All privileges must be earned through goody tickets, which are given to little boys who speak politely, mind their manners and behave appropriately. Whenever you catch your son in the act of doing something good, he gets a goody ticket. If he wants a privilege, such as a play date, tv time, an outing, whatever, he needs to cash in his tickets. If he doesn't have enough...oh, well! He'll need to behave better. It really becomes quite a game. It worked great with my eldest son; less so with my youngest.
For me, the "because I want to" response drives me bananas. My sons had lots of time in their rooms because my standard response was, "Oh, I see. I guess I want you to sit in your room and think about behaving better." Or, I would find a chore for them to do (yes, four year olds can do chores). My last best response to that was to decline the next play date or shopping request. "Hmmm...you want to play at David's. Well, I seem to remember that you did [fill in the blank] because you wanted to. Mommies of little boys who do that don't want to drive them to play dates. Maybe next time you'll behave better."
My sons are 12 and 16. They still, occasionally, pull that stunt on me. It never lasts long since I'm the chauffeur and I have no problem declining to drive them someplace. My eldest, when he was 13, decided to pull your 4 year old's stunt for a week (refused to do his chores because "I don't feel like it"). That Friday was a school dance and my son didn't go because I didn't feel like driving him to the dance. It's been 3 years and I haven't had that problem since.
Last but not least, I don't think there is EVER a good excuse for poor behavior. I would encourage you to not let your son "get away" with stuff just because you need to dedicate extra time to your daughter. When you have extra time, you'll share it with him. You might consider letting him know that and also let him know that you'll have LOTS more time to spend with him if he wasn't snooty and rude. After all, who wants to be with an unpleasant person?
In my house, the rule is that no one can infect the rest of the family with their bad attitude. Bad attitudes belong in one's bedroom until one can be a cheerful member of the household. That goes for mom and dad, too. Food for thought...
Good luck.