If you are playing this game for the first time, DO NOT READ THIS.

This is for people that have played the game a few times.

MEGA SPOILERS: To get the 4 main, different cut-scenes (3 different cut-scenes and one slightly different cut-scene). You do not have to replay the whole game.

All you have to do is replay from Fontain Futuristics.
Play the game from the start of the Fontain Futuristics level like this:-

If you decide to save Gilbert Alexander, Grace Holloway lives.
If you decide to kill Gilbert Alexander, Grace Holloway dies.

To get the 4 different endings, just concentrate on the 3 Little Sisters in the Fontain Futuristics level like this:

1. Rescue all 3 Little Sisters, save Gilbert Alexander > Good ending

2. Rescue 2 Little Sisters and Harvest 1 Little Sister >
Choose to die > Sad ending
Choose to live > Bad ending # 1

3. Rescue 1 Little Sister and Harvest 2 Little Sisters >
Choose to die > Sad ending
Choose to live > Bad ending # 1

4. Harvest all 3 Little Sisters, kill Gilbert Alexander > Bad ending # 2

You will need to have a saved game with the Fontain Futuristics level, saved at the beginning of the level. You can then keep replaying this level again and again, to try the different combinations.

I have not tried every possible combination, but it seems to work. You should at least get the 3 main cut-scene endings, by playing different combinations from the Fontain Futuristics level.

If you want to get every possible ending, you might have to replay the whole game from the start, using every combination?