A SR4 with a 40 or 50 on the back will do 32 to 35 kts (whisper managed 37 kts SOG with the tide on this years Nacho run), and at anything over 30 kts a SR4 feels very quick and exciting. A SR5.4 needs a 70 or 90 on the back, I use a SR5.4 with a 50 on the transom and it gets on the plane and does the job it's meant for, but it's by no means exhilarating.
A SR4 is really only a two seater, I guess you could make a SR5.4 into a four seater fairly easily.
Yes my 4 stroke is new so very fuel efficient. I was achieving around 1 litre per nm with my old 2 stroke 40 autolube
Rib Tigga an Avon SR4 Rescue with Honda BF50
MMSI 235910073 Callsign T123046