I make 4000 metres equivalent to 5700 PSI. You seem to have all the answers . 5000 PSI isn't too bad, its only two tons /sq. in. I've done work at 15,000 PSI, and had success with oil immersion. But my system was in a laboratory and not 4000metres away....

Fluid immersion works well, since there is no (or very little) energy storage, and little hazard. Our system worked in both compressed argon and also in a fluid. We made an induction motor to operate at 15000 PSI and 150C working in argon. The other end of the shaft was immersed in our sample. The rest is, as they say, proprietary.

If this is a "product", I'd look at vacuum potting for reliability over time, possibly using ceramic fibre reinforcement but you can forget maintainability. Fluid immersion (fluorinert ?) is great if you ever want to stand a chance of fixing anything.

Steve