Here's one way to do this. Create the following calculations:
CountA - Calculation - Number: If(Priority = "a", 1, 0)
CountB - Calculation - Number: If(Priority = "b", 1, 0)
CountC - Calculation - Number: If(Priority = "c", 1, 0)
CountD - Calculation - Number: If(Priority = "d", 1, 0)
Now, create summary fields to count these.
TotalA - Summary - Total of: Count A
TotalB - Summary - Total of: Count B
TotalC - Summary - Total of: Count C
TotalD - Summary - Total of: Count D