I want to marry all four marriageable characters and build a harem.

[ロリウム合金]

As the title suggests, true starborn should be loved by all.

[Loogie]

Start with Andreja. She'll get upset every time, but you can use persuasion to clam her down.

[zhdarkstar]

Like Loogie said, you can do this without a mod, but you are required to start with Andreja because she's the only one whose code actually checks if you're in relationship before entering one with you. The only thing a mod would be needed for is to remove that code from her so you can start your polycule with anyone.