How to clear a specific number of a specific item from a players inventory?

I am making a bed wars map and I wanna make it so when the player clicks a button, it give them 16 wool and takes away 4 iron. I have found out how to take only a specific item but how do I take a specific amount? For example, how I have it set up now, no matter how much iron the player has, it takes it all and only gives 16 wool. I want it to only take 16 at a time.