Is it possible you meant CS4-3 and CS4-4 rivets, not CCP-43 and CCP-44? If so, the CS4-4 is longer than a CS4-3 and is used when the material being riveted together is too thick for a CS4-3 to set properly. On the other hand, if the material you're riveting together is pretty thin and the plans call for a CS4-3, a CS4-4 rivet might be too long to set properly.
CCP-43/4 is the designation for CherryMax - N rivets (the "P" is for protruding head). The suffixes 3 & 4, indicate grip length in 1/16" of an inch - i.e. -3 = 3/16" grip & -4 = 1/4" grip.