Cover four, or quarters, is a tricky thing to nail down. Cover four differs from cover two and three in that it is pretty much always a pattern reading coverage. Pattern reading means that, in this case, the corner and safety are responsible for the deep half of the field, how that is done varies depending on the offense. Sure, teams can just have both corners and safeties split the field into four equal deep zones and have that be that but that isn’t really done all that often. The "four" in cover four comes from the fact that four players can wind up playing deep depending on what the offense is doing. There are at least a half dozen variations of quarters, but for our sake we are going to look at the most common form of quarters. Clemson bases out of quarters coverage, so take notes.
Let’s start with the simple pieces, regardless of which version of quarters being run, linebackers and defensive lineman have the same job. Defensive lineman and linebackers are responsible, firstly, for stopping the run. After that the defensive line (and maybe a linebacker or two) will rush the passer. The front, how gaps are assigned, and whether someone is blitzing does not matter to the secondary. Most of the time when teams run quarters there will be four defenders rushing the passer. The other three members of the front seven (or some linebackers and a nickel/dime back) will play three underneath zones. A middle linebacker plays, like in pretty much every zone scheme, the hook zone. He takes his read steps, drops back, keeps his head on a swivel, and tries to read the quarterbacks eyes while looking for crossing routes. The other two defenders will be playing curl/flat zones. First they look for any curl routes from inside receivers, drifting towards the flat as the play goes on. Curl/flat players are in no way responsible for anything going on eight yards or so past the line of scrimmage, the safety and corner have to deal with that. Curl/flat players also do not have force responsibility against the run, looking instead to make the tackle or force the ball outside instead of upfield or opening up cutbacks.
In cover four the corners will typically align somewhere around seven yards off the line of scrimmage, inside shade of the widest wide receiver. Some teams, most famously MSU, will line their corners directly on top of the wideout and allow the corner to play press quarters. Both have their strengths and weaknesses. Pressing opens a corner up to being beat deep but also allows more physical corners to beat up a receiver at the line of scrimmage. Press coverage also looks, on a pre snap read, a lot like cover two to the quarterback. Lining the corners up deep makes reading receivers easier as well as minimizing risk of a big play. Corners in quarters are essentially always playing man coverage on the wide receiver. This may be a zone coverage for the other nine guys, but the corners job is covering the number one receiver straight up. He will however usually have help deep, and this makes things somewhat easier.
Safeties in quarters align eight to twelve yards off of the line of scrimmage depending on down, distance, matchup and other factors. At the snap safeties will play flat footed, their eyes on the offensive tackle to their side of the field. If the tackle shows run the safety is the force defender (to his side of the field) or the cutback defender (on runs away). This allows quarters teams to have not just eight, but nine men near the line of scrimmage as run defenders. If the tackle pass sets the safety turns to read the #2 receiver (the second closest receiver to the sideline) and react based on what he does. If the receiver runs eight yards or so upfield the safety has the receiver man to man. If there is no deep threat from the #2 receiver the safety will help the corner cover #1.
Being a safety in quarters asks a lot. Safeties have to be able and willing to tackle running backs in the open field, cover a lightning quick slot receiver or a bulky tight end deep, and able to provide help against a teams best receivers. Oh, and safeties have to be smart enough to be able to know when to do which. Quarters asks teams to have two large, quick, intelligent athletes who can tackle at this position on the field at all time. Do not ever underestimate just how important safeties are, or how dang hard it is to play this position well. Every other position in quarters is protected, to an extent, and has a pretty simple job description. Doing this requires the safeties to pick up the slack.
Different varieties of quarters handle wheel routes differently, some ask the safety to pick it up, some ask the corner to pick it up and put the safety on the corners job, some even ask the outside linebacker to do it. All varieties require communication, awareness and speed. There’s a reason wheel routes are hard to cover.
What is quarters weak versus then? There are really two things that stand out. Regardless of how a defense plays quarters (and it can be as aggressive or passive as any scheme in college football) the defense will be ceding quick out routes. Ditto on bubble screens. The thinking here is that very few offenses can run those plays over and over successfully enough to move the ball, and even if they can most offensive coordinators will get impatient. Most quarters teams carry cover two in the playbook as an adjustment in case these little gains start getting too costly. Of course, doing this asks your corners to learn to be force defenders.
The other big weakness of cover four is play action, particularly to the #2 receiver. While it’s pretty hard to run on quarters teams, what with nine guys in the box versus runs and all, if a team can establish the run things get dangerous. If the safety overcommits to the run and the #1 receiver goes deep there’s not a single player on the field covering the tight end or slot receiver. Baylor has killed quarters teams with this for years. What are the counters to this? Honestly, the defensive line making a play or the safety not getting tricked/being fast enough to compensate. Deep play action passes are longer developing, meaning there's a longer chance for someone to disrupt the passer or for the safety to get into position.
At the end of the day, quarters is a simple coverage, there are ways to beat it, everyone knows what they are. Teams that base out of quarters are betting they can run their defense better than you can run your offense. For a lot of them, like, say Clemson last year or Michigan State for the last half decade, this tends to work out.