Stick with the solid floor 3.8 if four up. Anything smaller is too small. It will be ideal with two but with four a 20 or 25 would be preferable but that's a huge step up in weight and bulk to carry.
Again, if it is 90% of the time just two then a 3.4 airfloor (or 3.8 Aerotec) would be ideal matched with the 15. As always we're back to the 'which SIB' sticky but you have to choose according to 'your' specific needs (location/crew/storage/launch site/use) and then it come down to (airfloor) = lightweight/portable/easy setup vs (hardfloor) = heavy/slow setup/less portable but gives 'real' boat feel and better performance.