If 4 G’s was enough to cause fatal brain bleeds, then I’d assume stunt pilots, astronauts, and fighter pilots wouldn’t exist. And you can’t train your brain not to bleed.

G forces are good for measuring sustained force in one direction, relevant when you’re worried about fluids draining somewhere. Pilots are trained not to make an inverted turn that would be 4Gs towards their head, because that drains too much blood into your brain and will cause a stroke. 4Gs towards your feet or back on the other hand is no problem. You can’t just have some G number and say that’s safe.

NASA cancelled the Ares I rocket because even though the linear acceleration was less than 4Gs, it vibrated, and the astronauts all the way on one end of a 100 meter long vibrating stick would be getting jerked back and forth. Instead of excessive blood pressure popping a blood vessel, the threat was sloshing brain tissue stretching out and tearing blood vessels.

Looking at some video of X2 it’s pretty obvious how people would get a brain bleed. It’s trying to rotate people while they go around the track, and the hydraulics visibly suck and are shuddering and jerking like crazy. If people’s legs are flopping around like limp noodles, their brain on the other end of that rotation is going to be sloshing and twisting and that’s eventually going to tear something in somebody. The ideal design of the coaster was probably safe but unlike a fixed track where it either breaks or it doesn’t some 4D hydraulic system requires a lot of money to keep it ideal.