Yes the M4 can be set to clean the floor only. Only problem is you need the remote to do it. Otherwise it operates the default setting of floor and walls. The M4 doesn't come standard with the remote but my dealer offers it as a $200. option. That brought the M4 up to the same price as the M5 which I ended up getting.
I can only tell you what I know from researching cleaners before purchasing my M5 being that I don't own an M3. It looks like a decent cleaner. It does floor and walls but does not do water line. Like the M4 in can only run in default mode which is floor and walls. It does not have the 3rd roller so it looks like a likely candidate to get stuck on the bottom drain. Only an M3 owner with a bottom drain can tell for sure but all my Aquabots got stuck on the bottom drain.