Hi,

If I remember correctly it's something like :

• Step 1 : At the end of level 3, as soon as you validate 60sec press Esc twice. The first press will kill you, the second one leave the cutscene (if you press 3 time it exit the level, 4 returns to the main menu and 5 closes your game)

• Step 2 : Wait until "press space to continue" appears, press space or return, the game runs the credits

• Step 3 : Press Esc to quit credits, then right key, return, left key, return and then you are in the last level.

In my record I pressed Esc 3 time at the end of level 3. If you do this mistake you can re-enter in the level during the wait of step 2, it's a "backup" witch apparently doesn't loose time. That's why in my record, "press space to continue" appears on a green screen, not a black one. 

Hope it will help you.