Thank you for reading.
Which sounds more natural to native speakers, and could you suggest me some ideas on this?

1)He made generous contributions to the university that they are naming one of the new buildings after him.

2)He made generous contributions to the university, so they are naming one of the new buildings after him.

3)He made generous contributions to the university, therefore they are naming one of the new buildings after him.

4)Since he made generous contributions to the university, they are naming one of the new buildings after him.

@NK_Futures
Each of them is natural and has its use:

1. You are connecting using a relative pronoun with an intensifier meaning, so the preceding clause needs to be exaggerated.

"I'm not so sure I'll use it that I'd buy it".
"He's not such a bad guy that you'd kick him out".

2. "so" gives the meaning of similarity or equal weight. That one action results in the other as a natural course, and you are stating the fact.

"If you go to the theater, so will I", equally.
"I wasn't born into a privileged family, so when it was promotions time, they picked the rich kid", natural result.
"Your deeds were ill, and so you shall be punished", of equal weight.

3."therefore" is when you want to state a justification, a consequence.

"You didn't attend the class, therefore I will fail you", my justification (the reason why).
"He always says my cooking is horrible, therefore I won't give him any"

4. "since" (and because) are used to denote cause and effect, with since focusing more on the effect (because focuses more on the reason).

"Since you are at the supermarket, bring us some cookies", the focal point is the effect.
"Because of that I won't go", we can't use 'since', as the focus is completely on the reason.