I'd agree with those who said that post-flop you should raise a callable amount but one that denies him proper pot odds to chase if he's on a draw.

I didn't think raising with K9o on the button was such a bad play 4-handed, so long as you haven't been raising like a maniac pre-flop every single hand. You hope to pick up the blinds, and if you get called you know you're behind but you have position. I've certainly tried to steal blinds with much worse cards than K9o. However, make no mistake that K9o is a pretty marginal hand...if you get called you are almost 100% certain to be facing a better hand, if not dominated. Still, you have post-flop position and can take it from there.