Through years of observing boat test reports and then running a fuel computer, 3800 to 4400 RPMs was the 'sweet spot' for 2-stroke moters, with typically 4-strokes needing a little more. My bro can cruise comfortably @ 3600, but he has twin 150s.
When in doubt, 4K would be a good place to start. IMHO you're running fairly low, for a 150 on such a big hull ...