The E3 has a lighter barrel and the carry handle is on the barrel instead of the receiver. The flash hider is also different. The bipod on the E3 is on the receiver instead of the barrel and the bipod is lighter weight. The butstock does not flare upward on the E3 and does not have the plate that flips up over your shoulder. The top cover on the E3 is different allowing it to be snapped closed with the bolt in the forward position. The E3 also has a skinny handguard and forward mounted pistol grip. The main pistol grip is also a different shape. The operating rod on the E3 has two notches in it instead of one which allows the sear a "second chance" if it bounces out of the main notch. This reduces the chance of the gun running away.
The E3 Enhanced is the same except the handguard extends upward higher and the pistol grip is moved closer to the rear. The flash hider is of the duck bill configuration and the bipod is beefed up a bit. The flip up plate in the buttstock is added back as well.
The E4 is the same as the E3 Enhanced except a picatinny rail is added to the top cover and the feed mechanism is modified to provide increased belt pull characteristics.
These are the differences that I know of in the full auto guns. I don't have a semi, so they may be a bit different since they run off a closed bolt system.
In this picture, the gun is set-up in the E3 configuration but the E3 barrel has been removed and a standard M60 barrel installed.