There can be a vast difference in 4.0 averages. The hope with Elliott is that he's get 3.5/4 yards more consistently than Pollard did. Pollard was not a between-the-tackles runner, which is where our team wanted to run for some reason. If you get 1 yard and your next carry gets 9, that first carry is going to set up second-and-9 or third-and-9 and you are more than likely going to be trying to pass for a first down before the back is going to get that next carry. If your back gets 3.5 on first down, there's a 50/50 chance that the next play is a run too.
The running game was not good enough last year because it too often put us behind the eight-ball. Elliott doesn't have to average more than Pollard did to not do that, he just has to more consistently pick up positive yardage. I don't know if he can, but I believe this is what they are seeking from him. 4 yards can be effective when it is second-and-4 or third-and-4 and defenses can't just ignore the run. If it's second-and-10 or third-and-6, then defenses know a pass is most likely.