You can return to The Bull and finally beat the unicorn's riddle at the end of the game.
See the quickhints if you still need help getting past the doors.
First safe door: Second safe door: Third safe door: Fourth safe door:
When you get there, make sure to select the correct answer - '115'.
Door 4 (IV)
Door 4 (IV)
Door 3 (III)
Door 1 (I)