A 4" Bolivian ram would be an anomaly (which is not to say it can't happen- we once had a 4.5" Apistogramma borelli). They'll typically stay around 3" total length.
I wouldn't keep a pair in a 10 gallon, personally. Most people (myself included) can't sex them reliably anyway, so a store employee telling you it's a sexed pair is dubious IMO. A 20 gallon would be fine for a pair. If you absolutely have your heart set on using the 10 gallon, I'd suggest a single specimen rather than a pair.