I have a Janome serger (overlocker) it takes 4 threads, but can also run on 3 threads - this can be useful as a) you need less spools and b) if you are only serging the edge of one layer of fabric , 4 threads (2 needles) is a bit overkill, whereas, if you are using it for construction seams, 3 threads(1 needle) is not really enough. A useful thing to have on is the facility to do rolled hems for lightweight fabrics - not all of them have this as standard.
I haven't mentioned the model because it is 12 years old, and probably not available now. On the plus side for me, is that my 'normal' machine is also a Janome and they take the same electric cable, so I don't have to have yet another set of wires running about the place, as I do all my machining in the same spot, just swapping out the machines as I need them.