I had a period on March 22 - 25 and then on April 21 - 24." My period usually last for 4 days. Can a doctor please tell me when my next period should be for May?

You had 30 days in your last cycle. If you tend to follow the same pattern, you should begin your next period about 30 days from the start of your last period... May 21. It can be normal to be off a few days here or there however, but that would be a reasonable estimate.