mgm 120 bec

just got my mgm 120 today and put it in my rustler. everything went fine with the 3s lipo packs, took awhile to get used to the sudden reverse it has but overall i like it. thought i would put a 4s pack in it and do a speed run and everything went down hill after that.
i programmed it for 4s lipo, put them in and drove it around the shop for a minute. then i noticed my fans were running way to fast so i disconnected it. those fans smoke up fast! obviously, the bec failed me, or is it not designed to work with 4s lipo? the instructions only say 12 cells, which are not exactly equal but close.
so my question, is it not capable to use the bec funtion with 4s lipo? or is there a problem with mine? thanks