they both are multiplexing modes.
they show that you will combine 2 lines in 1 or combine 4 in 1.
if you have SDCCH congestion on abis with 2:1 you can change it to 4:1 and you have just doubled your E1 TS
2:1 is a half-rate multiplexing mode while
4:1 is a full rate concentrated multiplexing mode.
2:1, you can only add an optimum of 13 TRX for every 1 E1;
4:1, you can only add an optimum of 15 TRX for every 1 E1.
Considering you will fill every TRX with 1 SDCCH only.