What Makes for a Good Small Inventory System?

I am working on a small game, and it has five items you can pick up. Two of those are held items, and two others you cannot have at the same time (you use one to get the other).

So at most, the player will have four discrete items on them.

I was thinking of a gallery style inventory, but I don't necessarily want the player to know they missed anything. I don't want to do a grid of slots either, because all that space will never get used.

So far, I've narrowed it down to a row of 4 slots and adding in more items that the player can't use, but I don't know if those are good ways to deal with this. I could use some advice.