x4 mult if your played hand is the best possible poker hand you can play given your current hand.
By best possible I mean in terms of how poker hands are ranked in real poker.
For example, if you play a pair but absolutely nothing better was available, it'll still give x4.
However, if you play a two pair when you could've played a straight, this joker won't do anything.
For the hands that Balatro adds (i.e. 5oak, flush 5, flush house), maybe order them like this:
highest is flush 5
followed by 5oak
followed by flush house
followed by the other poker hands (straight flush, 4oak, full house, flush, etc.)
Perhaps the rarity or the x4 is a little off, but I do like this idea.