I'm surprised you feel so strongly about the 4th RB. I'd much rather find a way to keep Willis if it comes down to Dixon's spot. Personally, I think there are diminishing returns.
I'd also probably not keep Roberts over Dixon though...and if Dixon is so valuable, I'd trade him before I'd trade Carr/Smith (I don't think Jefferson is tradeable due to his cap number).
Well, I don't think Dixon should be the 4th RB but I do think this team should keep 4 RBs based on how they will play, having so many unknown RBs and Ingram being old.