Play Aggression

I am having some issues with my 4 1/2 month old kitten, Heinie. He is relentless in the pouncing, biting and play-fighting of his two older brothers. I did get Heinie when he was 8 weeks old and have read that this might be part of the problem in that he didn't learn "proper" play from his siblings.
Pirate is my oldest kitty and is almost 4, Vinnie just turned 1 in June. They do seem to like the little guy. Pirate grooms him and Vinnie is his usual playmate. I have noticed lately that Vinnie is seeking me out for protection and this worries me that the inter-kitty relationships could be damaged if the aggression continues. The big kitties do occasionally hiss, smack and generally put him in his place, but I don't want them to get stressed while we are waiting for little "atilla" to grow out of this phase.
I know that Heinie needs lots of playtime to wear himself out and I am starting to think I might need to get another kitten close to his age for him to play with while I am at work. If I get another kitten that will raise me to 4 cats... have any of you dealt with this challenge (play aggression) and can you give me tips on how to "protect" the big kitties? Is getting another kitten a good idea? I am happy with the number I have, but can always love one more... I just don't want to multiply Pirate & Vinnie's persecution: )
Help!