The "low revs extends engine life" is a common myth. Loads and stresses on the engine can be calclated and usually peak about 60-70% of peak rpm. The math invoved and simulations are very complex but rest assured that Rotax have done this.
Another way to think about it is that is that if you need X power then X/5500 is smaller than X/4200 so your engine is more stressed at 4200 rpm then 5500.
The old term for this is "lugging".
Short answer it is better for your engine to cruise at 5200-5400 than well under 4800 rpm.
You think you are being "nice" to the engine but that is not the case.
