The difference is that some props are designed to revolve twice per power cycle while others expect a power cycle every revolution. OK, that is complete BS. There really isn’t any such thing as a 4C vs 2C prop… it is just marketing. However, there may be prop designs that lend themselves more toward one engine type. 4C engine tend to produce their power at lower RPMs compared to 2C. This RPM/power curve difference does lean toward slightly different ideal designs for 4C vs 2C props. But even so, any prop with the proper loading will work on any engine, which is why in general you will never see a prop marked as for 4C or 2C as they work fine on either. Cheers.