Whitneyblayne,
He's almost 4 and driving you crazy. My advice, after making sure it's nothing medical, would be the same as young br_hjs. Sounds like you may have to be more firm. Set some appropriate rules and boundaries to suit his age and stick to them. Decide on punishment before hand; taking toys away, time-out chair, no sweets, early to bed etc. Be firm and consistent. I don't believe it is to do with being an only child. But he may be jealous of the attention you give the other children, especially if you normally pay him a lot of attention when you two are on your own.
If you feel like filling us in a little more, we might be able to offer more advice.