If your supplier has the item as $1 per unit, but it is sold as a 4 pack on Amazon. You would just multiply the cost ($1) by the amount of units in the pack (4) to get your cost per pack, which would be $4 in this case.
You would then use that cost as your purchase cost when you enter it into the FBA fee calculator.
My supplier is giving me a CPU price but on Amazon they are sold in a pack, how do I get my profit?
Can I calculate my profit if I'm selling an item in packs but my supplier only gives me the CPU?