The greatest asset of the 4-2-5 is its flexibility and that comes in handy when defending 11's personnel.
In my eyes, there are three basic 11's personnel formations you will see:
#1. Pro Ace
#2. Pro Trips Ace
#3. Pro Trey Ace
Of course there are plenty of variations with receiver and back alignment, shotgun, pistol and other looks, but we'll stick with those three.
To simplify things, I will line up and play base defense in one of two ways:
#1. If I want to stop teams that run predominantly to the TE side, I will play an Under front and Cover 3.
#2. If I want to stop a strong passing attack, I will play an Over front and Cover 4.
Let's assume we are in the middle of the field
We'll start with what I call Pro Ace.
We call our Over front "Tight", which tells our (M)ike LB to set the front strength to the TE side. Across the front, we are in a 6i, 3, 1 and 5. The (F)ree Safety will set the secondary strength to the passing strength, so he will make a "Right" call here. Both our F and (W)eak Safety will make a check to their side based on their rules. With a #2 detached to the free side, we make a "Read" call and play 2-Read. With 1-back and 1-WR to the weak side, we make a "Read" call and play 2-Read.
When we play Cover 3, we set the secondary strength to the TE side when one is present. This allows us to put our $, who is usually the better run defender out of him and the W, to the TE side. He is usually a better player for us at playing on a TE as well. We play Cover 3 with similar rules to Saban's Rip/Liz, with some notable differences, but our $ and W will carry the vertical of #2 until #1 comes shallow. Our F splits the #2's (here, the TE and slot WR) and plays high hole.
Moving on to Pro Trips Ace...
We use automatic trips checks for different formations. I won't signal in that we are going to play Special Sky, we will just game plan it that way. I just signal in we are playing Cover 4 here. Again, we are in an Over front and the F is setting the secondary strength to the 3 WR side. We run Special by playing man-to-man on #3 and playing 2-Read to #2 and #1. On the weak side, we play Sky.
Again, when we play Cover 3, we will set the secondary strength to the TE and our F will adjust if he needs to. We are in "Split Tough" (Under) up. Our F is now taking the vertical of #3 to #2, the W is carrying #2 vertical and our (C)orner has the vertical of #1.
And lastly, Pro Trey Ace...
With a Trey set (TE and 2 WR's) we will check to "Solo" in Cover 4. We do this because the run support is good to the TE side and our W can handle the vertical of #3 easily. We play 2-Read to the #1 and #2 WR's to the free side and our weak C has his WR man-to-man.
This is the bastard formation for us and what takes us out of our Under front. Our front is set away from the TE and our secondary is set to the TE side (remember, we do this in Cover 3!) Because we didn't make a "Tough" call, our $ plays in his Cover 3 alignment vs. #2 detached, our H goes to a 6i with a TE on his side and he makes a "G" call to get the N into a 2i.
For us this is pretty simple and flows seamlessly with the rules we have up front and in the secondary. Of course there are many other options! We will play our Over front and play Cover 3 sometimes as well, but this has worked very well for us. Sometimes simple is better!