I really like Cover-4 in spots. It's a very solid run defense, numbers-wise. It sounds like you're dropping 4 deep in the coverage, but the way I learned it from CJT and other coaching clinics back in the early 2000's, it's not how it sounds. It effectively gives you 9 in the box. You've got your front 7. Then whatever is the playside on a run-play, that safety plays the alley while the backside safety remains in coverage (maybe on a particular man, maybe going to the deep middle - like cover 3). Safeties play at about 9 yards deep so they can be there quick in run support. We used this to great effect in the CJT years. Auburn '05 comes to mind. It's got some similar principles to the old Tampa-2.
Here's Narduzzi's version if you REALLLLLY want to know more:
http://www.championshipproductions....-with-4-3-Over-Cover-4-Defense_FD-03922A.html
I wish I could find an old coaching clinic book I had that explained the version I used to run. If I had to just pick one defense to stick with ALL the time, that would be it.
Something I'm totally ignorant on though now is the RPO offenses (so common now in NCAA) and the strategies to defending them. That wasn't really a thing back in my day (like 10-15 years ago).