I might be wrong about the card (4 of spades - Gomorrah) it could be a 4 of clubs or something else entirely but I hope that's enough to give an idea of what I'm asking.

I've found these cards and, IIRC, I can't get rid of them...I might be wrong about that too but the point is are these cards used for anything?

They're for playing caravan.

How do I add them to my deck?

You cant if you are playing on the PC. They are bugged.

EDIT: Just read that this issue has supposedly been fixed in the latest PC patch. In order to add them to your deck transfer them to a container and then take them again.