Fluxx - What counts as a 'play'?
One thing that may be worth clarifying (for new players) is that, if you've played one card already this turn and your second play is "Play 4", you don't play four more cards; you play two more cards.

Likewise, if you start your turn with "Play 4" in effect and your third play is "Play 2", you don't play any more cards; you have already played more than your currently allotted number of cards for the turn.

Playing cards as part of an action doesn't count towards your Play 4 limit, though.  For instance "Draw 3 and play 2 of them" counts as one of your four plays for the turn, not two or three.

Playing a Keeper or a Goal does very much count as playing a card.  Creepers don't, of course, but they're a well-documented special case.