You will have no trouble finding a SR4 as they are always about, the 4.7's are much less common so if you see a good deal on one don't hang around.
They are quite different though and with a two man console on a SR4 there is very little deck space. No finer boats for your first RIB though. Run a search for 'searider' in title in the RIB section as there is tonnes of advice on here.