I think it's something like this but I'm not 100% sure
R= (Verizon/Sprint) 1xRTT (basically 1G speeds)
E= Edge (AT&T, TMobile, basically 1-2G speeds)
G (alone) = usually UTMS (AT&T, TMobile, basically 1-2.5G speeds)
3G (Verizon /Sprint = EVDO Rev A, AT&T/TMO = GSM/HSPA)
4G HSPA+ or LTE