I always thought it was 4400 - 4800. 4600 -4800 is a tight target. Either way, 4000 is over propped for the weight. Maybe there are too many people or gear on the boat. Typically these boats are spec'd to run basically empty with 1/2 load of fuel and water (if there is additional tankage). Max RPM start to creep down as 'stuff' and people start getting loaded.
Run the boat at WOT, with as little trim tab as possible. Trim the drive out one click at a time until it cavitates, then bring it back in until it stops. Speed is irrelevant at this point, as is running with or against current or tide. Those factors only impact speed, not rpm.
Either weight needs to be lowered or the pitch needs to be lowered until you're in the 'sweet spot' for RPM. Prop based on the typical loads (people, gear, coolers) etc.
Avoid the thinking "I want to run with 8 poeple and perform well". Be honest with yourself and the expectations and reality of actually running with a large group of people.