Is polygyny the norm in Islam, or is the letter "fa" in the Almighty's saying, "then marry" [Quran 4:3], causative?

The "fa" in the Almighty's saying: "then marry" (fa-ankihu) is the "fa" that links to the apodosis (answer of the condition) and indicates causality in the verse. Its meaning is: If you fear injustice towards orphan women, then marry from other women. It is permissible for one who does not fear injustice towards orphans to marry four women if he wishes and knows he can be just. However, if he fears injustice, then he must limit himself to one.