Most coaches will want safeties to play outside linebacker and speedier guys at all spots on the field. On teams that use the 4-3 system, bigger, slower defensive lineman are required to play offensive line. They need to be able to handle multiple positions because there aren't enough healthy players on the team to fill them all out. That's not to say they can't also play defense. It's just that when the offense runs plays designed for the big men up front, they have to be able to take some hits too. That's why you'll often see them wearing helmets with face masks attached to protect themselves while still giving off an aggressive appearance. The 4-3 system is very simple to understand but difficult to master. If you can recruit quality players into your program, you can run it effectively. The key word here is "recruit". You can teach anyone how to fit into a 4-3 defense, but it takes someone with talent to actually make the switch from offense to defense. Coaches looking for defenders who can cover tight ends and receivers downfield will want to look for players with this type of skill set. They can help out by playing inside at tackle or even center if they're good enough athletes.