The M4E1 VS AR15 receivers are functionally the same and the only differences are design and small ergonomical features. Ultimately, all of them can work with standard lower parts kits.
As far as upper receiver goes, the M4E1 offers two versions. The threaded upper receiver and the enhanced upper receiver. The enhanced upper uses a one piece mounting system for the handguard without using the barrel nut. This is more rigid to mount a free float handguard system to increase overall accuracy.
Aero Precision M4E1
The M4E1 Lower Receiver is a special part we make that looks cool like a fancy version but is as strong as a regular one, and it doesn't cost a lot!
The magazine well is flared so the magazine can go in much easier than a standard AR15 lower receiver. The trigger guard is also built in as a part of the receiver, so it's easier to use, even with gloves on.
For a traditional AR15 lower receiver, the user has to use a 1/16" hex key and a roll punch to get the trigger guard installed. The M4E1 doesn't require that.
Instead of using a tricky pin to put the part that catches the bolt in place, we use a simple screw. This makes it easier to put together without needing special tools or worrying about scratching it. We give you the screw when you get the M4E1.
And lastly, the M4E1has the 90 degree fire selector marking as well as the 45 degree fire selector. The Radian fire selectors are highly recommended for a build like this.