There is no joy associated with mixing DOT 4 and DOT 5. If you don't change the rear MC, line, and caliper, you will have two different brake fluids to deal with. Good luck.

 

I just swapped out the front calipers on my bike. I got a set of take off calipers complete with brake lines. I used my old master cylinder, however, and the old rotors. If there is a problem with mis-matched parts, I haven't encountered it yet. (not to say I won't in the future, though). I guess I don't have the increased brake line pressure since I'm using the older master cylinder. I'm also using my old rotors, and so far it works just fine.