Hi EBB,
There are several Rotax mechanics around the San Diego area.
You should cruise above 5000 rpm. (5100-5300 is pretty good) It can run at 5500 rpm its entire life. The engine was never designed to run in the 4000's for extended cruising. A little slow flight scenic viewing is fine, but shouldn't be for the life of the engine.
The higher temps are coming from an over loaded engine from a prop pitch set too course at 5200 rpm at WOT. Flatten that pitch by about 1.75 - 2 degrees and get it up around 5550-5700 rpm in flat and level WOT. 5600-5650 is a good place to be for a ground adjustable prop. Too course a prop pitch cause high temps and engine over stress. Some engines have developed cracks on top of the crank case from this. Getting the rpm up to a more normal range will give you cooler temps, less engine stress, better climb, better cruise and top end and better fuel economy.
Read this and it will help:
http://www.rotax-owner.com/rotax-blog/item/9-understanding-the-ground-adjustable-prop
See these sites for help:
http://rotaxirmt.com/
http://www.rainbowaviation.com/repairman.htm