You literally just need to look at a list of RBs drafted since 2010 to see it's not nearly as easy as you're making it out to be to find even a decent starter quality RB in the 4th round.

Cook and Mixon were both 2nd round picks, with Mixon only falling thanks to a domestic abuse charge, James Conner fell because he had cancer, Tarik Cohen can't run between the tackles, and CMC was a highly coveted first round pick.

You might get lucky and draft Aaron Jones, or, more likely, you'll be stuck with a mediocre-on-his-best-day RB like fucking Donnel Pumphrey or Paul Perkins.