If they're your best units, then you gotta keep 3 around or so. You could conceivably rely on your allies for the heavy lifting as outside Colo / Arena / GW you don't need a full team (I think).
Right now 4* feeders are around 1 TE / Pot a piece, with some people selling as low as 3 pure for a set.
So ya, the value of 4*'s is pretty low. However, that means the value of 5*'s isn't all that much better (and you can see people doom-glooming the death of 6*'s too).
While I've had trouble selling Gods / leveled 4*'s for pure, I have had luck trading them for 5*'s. I don't know if 5* sell easier, but at least I have better units to work with.
Speaking of which I should start selling those 5*'s now that trade lock is over.