In a keeper league, Green and CJohnson seem to have the most value. Johnson could very well be a top twenty back for several years and that could solve your WR problem. That being said, with a premium on running backs, you have to keep the pick and go with Barber or Green if they're still available. Wait until the first top tier running back goes down this season, and then dangle one or the other in front of the owner for a top ten receiver.
If they're both taken by number four, which is a strong possibility, I would take Johnson.
It's time to roll the dice!