If you spent those 4 cubits (turning them into a gift card), but only had to use 3.50, those 0.50 cubits left on the card would remain on it. You wouldn't get them back onto your Minecraft account, but you'd be able to use the remaining balance on the gift card for any future purchases you make with other gift cards or actual money. Just in case type /cubits giftcards when in-game, and if an amount still shows up on that gift card you used, you should be able to use it again.