It also would change game plans. If a team KNEW they had 4 downs, that would open up to different and unique strategies. It definitely could work, there is numerous HS teams who do it and have tremendous success. If some of these elite offenses had 4 downs, all they have to do is average 2.5 yards a play for a first down. I think its worth an attempt, but I don't think any NFL team is ballsy enough to attempt this. Teams don't like going away from traditional strategies.