How many weighings you need:
Call the four coins A,B,C and D, and the true gold coin G. You start by weighing
AB | CG
If this balances, then it is only possible that D is defective. Weigh D against G to find out for sure.
If it tips towards AB, there are three possibilities: either A or B is heavy, or C is light. To figure out which one, weigh A against B. If they don't balance, you learned which of them was heavier. If they do balance, you learn C was light.
If it tips toward CG, this is very similar to the previous case. Just switch the words "heavy" and "light" in that paragraph.
Furthermore, it can't be done in just one weighing. There are nine possible situations (each coin could be heavy or light, or all could be same), and a weighing has only three outcomes. By the pigeonhole principle, there will be some outcome which would result from from two different situations, so a weighing could not distinguish between those situations.