If you're sure he has kings, then you fold. but assuming you played on, then it is probably usually better to wait until 5 or 6th until raising if you are confident he has the higher pair. Remember, that your raise is not going to chase the higher pair out, so since the raise will not have that power, you want to be sure that you are indeed raising with the better hand. wiaiting until 5 or 6th allows you to see if he gets a pair on board. without that pair on board, you two pair is likely best and your raise now will get your more than if you raised on 4th. waiting till 6th gives you the opportunity to see how his hand plays out.

however, if you doubt that he has the higher pair, i might go ahead then and reraise on 4th trying to get him off of his hand right then, not giving him future drawing chances. I know others would still slowplay here trying to extract more money, but you are paying for that by continuing to let him draw cards which can beat you.