It depends. There are overlocking machines, coverstitching machines, and combination machines that do both. In order to be able to do a 5-thread coverstitch, you have to have a 5-thread machine that's either a coverstitch or a combination machine. (5-thread coverstitching is something I'd have to have, as this is the primary hem stitch for t-shirts and many, many other knit fabric.) A 4-thread overlocking machine cannot by definition do any 5-thread stitch.
You might have meant that if I have a 5-thread machine, why would I need a 4-thread. Switching from one to the other on one machine is a real pain, from what I've read. (I have yet to try to do this myself but in reading the instructions on switching from rolled hem to overlocking on my Baby Lock machines, I would hate to have to go through all 14 steps each time I need to switch from one to the other.)