I believe with a jcm800 running 1 cab, the ohm output is 16ohms, while a mesa 4X12's input ohm rating is 8 ohms.. so that won't work. You could get a weber z-matcher, it's a device that corrects ohm mismatching. You can get one
here . They're $80-$100, depending on the wattage of the marshall. Good luck buddy, jcm 800's ROCK.
edit: jalin, is your avatar from a Tool video? It looks familiar. I see you also have a silverburst lp... lucky dog!