Checking the position of entities

[dragonlordsd]

the basic idea is this:
the player places four skulls (entities, but not items) in the circles on the alchemic formula decal. Then by applying a formula (an inventory item) to one of the skulls (since they're so close together, it can be any one of them) it causes a key to appear.

Just to make it easier, I want a function that, when activated by using the item on the skull, checks if there are four skulls within a set area. They don't really have to be positioned exactly, that would probably make it too annoying to solve.