I need someone to check this: go to the point on level 4 indicated on the attached screenshot. Make sure that (a) you're not using the WHDLoad invulnerability trainer and (b) you're not in the game's own "flashing invulnerable" mode when you hit that spot. If I'm right, the Speed Of Light subgame should start.
It looks like the game isn't bugged after all, apart from not being able to enter the subgame while "flashing invulnerable" (or using the invulnerability trainer)
It might be that the subgame has just remained very, very
well hidden until now!
It's like I always say: "Life would be so much simpler if I had the source code"