What would be the advantage to putting some pistons in an F4i? And what kind of gains do you think I would see? Thanks

 

You generally only change pistons if you want to change the compression ratio or if you need stronger ones to make more power. Unless you plan to turbo your F4i or do major engine work I wouldn't even bother. If you just swap pistons it will be a lot of money and effort for no gain.

 

Would be pointless and a waste of money. spend the money on porting your head if you want to see gains.