I don't think that there would be much difference between towing a 4.2 or 4.8m RIB. In good conditions they will both be fine, in heavy weather they will both be a liability!
There are a couple of RIBs available that can be dismantled and stored aboard, but they are much smaller than you are looking for. For example the CARgoCRAFT
is a clever design, but it's only 2.7m long. Henshaw make the Tinker Foldaway RIB
which is a very cool little boat, but again its only 2.75m.
If you decide not to tow a RIB, then the next best thing would probably be the Zodiac Futura Fastroller MkIII
which is 4.7m long. It has an inflatable floor so it is easy to assemble -- you won't want an inflatable with rigid floorboards if you're going to assemble it on deck! The ride won't be as good as a RIB, but it would make a reasonable compromise.