The cornerbacks generally line up >7 yards off the widest (#1) receivers and play man coverage if they attack vertically further than 8 yards. If #1 releases underneath or runs a quick out-breaking route, the corner must gain depth and play a deep 1/4 zone.
The safeties in Cover-4 read the #2 receiver (can be a slot receiver, TE, or RB). If #2 releases vertically, the safety is in man coverage. If #2 breaks his route prior to 8 yards, the safety doubles #1 from the inside. Safeties in Cover-4 often have run-game responsibilities as well, either "force" (i.e. set the edge) or cutback.
The underneath players in Cover-4 have a great deal of ground to cover. The two outside defenders defend any route into the flats while the middle player plays the middle hook and "walls off" any route underneath. The flats are a weakness of Cover-4 because slower players have a lot of ground to cover.
Outside References
1) Inside the Playbook: Michigan State's Cover-4 Defense
2) NFL 101: Introducing the Basics of Cover
3) Read Call - Backside Safety Support
4) How Troy Polamalu and Ed Reed Changed NFL Defenses
5) Get Back to Fundamentals: Coverages
6) Breaking down the top Cover 4 beater
7) Playbook: Broncos' Cover 4 beater vs. the Saints
8) Film Room: AFC Divisional Round
9) The Quarters Coverage Study
10) Quarters Coverage
11) Press Quarters Coverage
12) 3 Insights to Improve Quarters Coverage
13) Quarters Coverage Alignments
14) Stanford Quarters Coverage vs. Oregon
15) Inside the Playbook: Cover 4 Safety Play
16) Quarters Coverage from Football-Defense.com
17) Quarters Coverage versus Pro Set, Twins, and Trips
18) Quarters Box Concept Versus Bunch Formations
