During the early stages of GTA 6's development, Rockstar considered featuring four protagonists but ultimately abandoned this direction. Rob Nelson, co-head of Rockstar, explained in a recent preview that having too many main characters could make it difficult for players to form emotional connections with them.
Regarding the specific plot of the four-protagonist version, there is another unconfirmed leak on GTAForums. The leak claims that the four playable characters included Jason, Lucia, and two corrupt police officers. Midway through the game, a decision made by Jason and Lucia allegedly leads to the death of one of the officers; the other officer then pursues the pair in the latter half of the story.
Concerning the script adjustments, reports indicate that Rockstar felt this version of the story was too crowded and convoluted, making it difficult to follow. They later rewrote the script to focus on the duo dynamic between Jason and Lucia, summarizing this direction as a "Bonnie and Clyde"-style narrative.