hi i have a marshall mg dfx 100 combo, i want to run this into a 4 x 12 cab, but can i do this and if so what cab should i use?
If it has a Line Out, then sure you can. Any cab will do for the most part as long as it is rated for 100 watts. Tube amps work a little differently. You may lose the internal speakers of your combo though. Not sure.

If it has an extension cab output you can do it. Don't use the line out straight into a cab, that wont get you anywhere.

But really.. get a new amp instead, don't worry about a new cab. a nice 30Wvalve amp would rip that MG to shreds =]