Normally, four backs might be an unlikely thing.
Not with this regime. This regime believes wholeheartedly that when you have good backs, you keep them...because you're going to need them. They value runners heavily. They don't believe in drafting them high in the first round, that's not their thing. But, they do believe in drafting them often until you have a stable of horses that you know can run.
Right now I believe Patrick Cobbs IS the #3 back, and Jalen Parmele has his work cut out for him if he wants to get the job back. Cobbs even working with the first string some. Bottom line is he's extremely well rounded, works his @ss off, and is doing everything they ask him to do really well...catching, blocking, running, blocking kicks and punts, playing gunner on coverage units...forget about it. You keep a guy like that.
Incidentally Ed Miles is also making this team for much the same reason. Tireless worker that has a knack for excelling at a number of different jobs they ask him to do. Camarillo too.