The M4 trigger body has larger disconnector spring pockets, so requires larger-base disconnector springs. The M4 burst cam is shiny and is referred to as a "cam, control" instead of "cam, burst", and even has a separate part number for its torsion spring.
I can only speculate on why the different springs - maybe different spring rates to improve trigger feel in semi-auto?
The full-auto M4A1 takes a variant of the old M16A1 hammer (hammer, small) and hammer spring, too.