VP Duoprops are numbered from 1 to 7, with 1 being the least amount of pitch and 7, the most. If you run A-4s now, can reach WOT with full fuel, people and gear, then you are propped correctly. If you over-rev, you need to go up in size. If you can't reach WOT, and the engine gets hot (fully loaded), you need to go down in size/number.

A series has a 3-blade large front prop, and 4-blade rear smaller prop.
B series has 3-blade both.