I loaned a person $1500 4 years ago at 20% interest compounded monthly. How much money is owed me today?

Have that not paid anything?

If so, the formula is PV * (1+i)^n

PV is the present value, I is the periodic interest rate and n the number of periods.

1500 * (1+.01667)^48 = 3316.37

If you have a spreadsheet (like Excel), the FV function will do the calcs for you.

I loaned a person $1500 4 years ago at 20% interest compounded monthly. How much money is owed me today?

Do you mean that the interest rate is 20% per month, or per year? Depending on that answer, and what state you are in, it could be "nothing", because you are above the usury boundaries in many of the states. (A quick Google for usury limits shows that many states put them at around 16%, and many have it at less than 10%.) 20% per month is definitely usurious except in the very few states where there is no usury limit, and even there it could attract legal attention, because it looks like loan sharking. And the interest rates cited in the usury laws tend to be simple, not compound.

This, of course, assumes that you are not a business. If you are, then I would think that you have an accountant that could tell you that. However, strictly as a math problem, a Google for compound interest calculators gets an answer of 3316.37. Of course, that's simply what you get when you plug the numbers in, and doesn't count any repayments that the person has made. That would get to be a messy calculation, depending on how much of the payment went to paying off interest versus principal, etc.