As of 2024, the situation regarding the "4 player mod" is different depending on which platform you are playing on. It is crucial to understand the distinction between the PC version and the console versions (PlayStation, Xbox, Switch).
The existing 2-player netcode is notoriously buggy. Modding it to handle additional players would likely cause massive desynchronization and save-file corruption. End-Game Triggers: stranded deep 4 player mod