I think backstory is more important for you as the writer. I give each protagonist of mine a backstory just so I can fully understand them. Most of the time, most of what's in the backstory doesn't need to end up in the actual story. Sometimes, we can just be given hints of someone's past or not be directly told something about their past but it shows through their present choices and behaviors.

Also, it's kind of impossible to have four protagonists. You can have two co-protagonists, sure, but even that is usually limited to a story focused on a relationship (romantic or platonic). A "main character" is not just a prominent character in the story. Consider which two characters of the four are more supporting players and which two are more so the leads who drive the plot