https://www.youtube.com/watch?v=Zz95_VvTxZM
Video contents: An experiment where the vacuum safety valves were removed from an oil tanker railroad car, and a vacuum was placed inside the car, resulting in it crumpling in an implosion. There is no gore or other specific disturbing content
"almost 4000 meters of water" is "almost 400 atmospheres". About 2.7 tons per square inch. I find that that absolutely unimaginable. - I can't find any equivalence that my brain can make sense of.
For every ten meters of descent, pressure increases by 1 atmosphere. At 4000 meters, that's 400 atm, or ~40 Mega Pascals, which is significantly higher than the pressure caused by the bomb that leveled a building in Oklahoma.
The hull shattered catastrophically, and the pressure wave liquified the occupants instantly. It's horrifying, but at least it was painless.
Probably the fastest and least painful way to die, short of sitting on a 2000 lb bomb or the like when it goes off.