When I was flying a Swiss SE Turbine Adelaide to the island was a FL120 leg, or even up to FL160 if it was off Rwy05 and onto the Rwy01 RNAV. Rex does it at 8/9000 typically in the Saab.
4000ft (even with a life jacket) isn’t ideal but then a lot of the time they go down to 2200’ overwater still and practice holding (inc at night). The hardest part is managing them as traffic - usually the students second language English radio calls are better than the South African instructors botched attempts.