I don't want to diss the SIB brigade but having just got a cheap 3.2m SIB my thoughts are....
1. It takes a hell of a lot longer to load up a SIB, unload it, put it in the water, mount the engine on it and get everything up and running, than it does to launch a RIB on a trailer and be ready for the off. And that's not including putting it all together and inflating it which probably took me about an hour the first time I did it, I am sure now I know what goes where the next time will be a lot quicker but even so it is a LOT more faff than a boat sitting on a trailer ready to roll. My old RIB was always fuelled checked over and ready to go so if there was a window for a quick fix, grab the bag with lifejackets and radio in, hitch up and off, I could be on the water and away in about 20 minutes from leaving the house.
2. Half as much boat in length is in no way anything like half as much boat in seaworthiness, comfort, speed, range, equipment or anything else.
3. Seven days and counting till the Vipermax arrives and I can't bloody wait
I really hadn't appreciated point 1 till I did it, you tend to think that its a 5 min job and will be "great for the odd quick spin in the winter" until you actually do it - especially single handed as I did.
Having now tried both I'd not ever consider downgrading to having just a SIB and you certainly wouldn't catch me out in mine in a F4. OK mine only has a small engine (the new aux for the RIB) so I didn't expect much speed but I think if I put a 15hp on it I'd probably need new underwear - I suppose confidence might come with more use but I just didn't feel like I'd want to be out in any sort of waves in mine whereas in a RIB that is only 50% bigger (3.2-5.8) it feels like twenty times the boat. I guess the difference between 4.7 down to 3.8 is not going to be quite as much but I bet it is still a hell of a lot
I don't regret buying the SIB as it was stupidly cheap and I have plans to use it somewhere else, but if I had downgraded to save money I'd be very disappointed.