Probably Counterfeit Masses

So, time for the final counterfeit masses problem. Usually I say where I first heard my puzzles from but I actually made this one up myself, with Matts help:

You have a balance scale and 41 coins. Most of the coins are identical but there might be a counterfeit among them. The only difference with a counterfeit coin is that it has a slightly different weight than other coins. One of the 41 coins is set aside, that coin is promised not to be counterfeit. You may use the balance scale to make 4 measurements, find an algorithm that is guaranteed to identify which coin (if any) is the counterfeit coin and if it is heavy or light.

So, its essentially the same as the more counterfeit masses problem in that you don't know if the counterfeit is heavy or light, but now you don't even know if it exists. To make up for it, you are given a single coin reserve. Apparently that is enough to cancel out not knowing if the coin exists.