With a short hull weight is critical or you will upset the balance of the boat, hence our recommendations for 2 strokes. That Merc 50 hp is likely to be 1970s. The reason I said don't go to Bill is that he is very expensive and really only as a last resort.
The 3 cyl Yams and Mariner/Mercs will command a top price however this would do nicely. Okay its hand start but pull starting a 40 hp isn't difficult and means you can then get away with using a small motorcycle battery (saving space and weight) for electrics like a radio etc. For the price a late 90s engine will be much more reliable than a 30 year old Merc. Bembridge are members of Ribnet
and built some very nice Shearwater cabin ribs. Having bought covers from them and visited several times I would be happy to recommend them.
Is 40 hp enough for a 4m rib? yes, I think that Merc 50 was rated at the powerhead also any wear will have reduced the power output, whilst the Mariner 40 will be at the prop,