There are a couple ways to calculate this. You could do it step by step, subtracting 1/5 of the total mass each time, but the easier way to do it can be done in one step.
Subtracting 1/5 of something means leaving behind 4/5 of it. 4/5 of something is the same as 80%, or 0.8 times that amount. So, each step of subtracting 1/5 is really just multiplying by 0.8. So 4 minutes, or four repetitions of subtracting 1/5, looks like this:
50 * 0.8 * 0.8 * 0.8 *0.8
50 * (0.8)^4
This value can easily be calculated as 20.48. So after four minutes, the substance will weigh 20.48 grams.