I dunno. Have a blank box space next to your scoops so you can put in the number of scoops you want to trade in. If it’s not a multiple of four either use a pop up error message or just show the remainder at the end of the card selection results. Then press”trade”.

As for choosing the cards… just eliminate that entriely for this method. Have it pick the cards at random (not likely they will all be the same). My reasoning is, if they really want to pick their choice of cards instead of letting the computer choose automatically, let them do it the manual (long) way.