This is getting a bit too involved for me. I don't want to make decisions for you that may or may not suit.
Using 4 of the new panels at 48v with mppt would be fine, if you can sell the old ones at a reasonable price it has something going for it ( how long are they going to last?)
I have no idea of your cable run, 140 ft is not that frightening if the cable is sensible. I think this cable loss issue is not as bad as often thought and sometimes the extremely heavy cable people use doesn't make much economic sense, the issues are different with a current source compared with a voltage source.
If you stick at 12v with the mppt then it will certainly be more efficient at 48v than 24v but whether you will be better off with 4 70W panels or 3 and the existing ones then I really don't know, I wouldn't expect there to be much difference. Depends on the relative cost of another 70W panel compared with what you could get for the old ones.
If you go the new panel route you might consider 3 70W connected in series as nominal 72v rather than in parallel for 24v. If you use 4 new ones then series again should be within the rating of the better mppt controllers and would be better than series parallel at 48v.
Flux