My son have a Colt LE6920 M4 carbine, and I have both the model CE1000 Expanse M4 and AR6721 AR-15A3 Tactical Carbine.
I just got my Expanse carbine about 5 days ago and I've actually got two A3 Tactical carbine, my older I changed my barrel with a limited run custom midlength gas system, medium contour, 16" barrel and my newer is factory stock with HBAR heavy contour barrel under the handguards.
Most of the difference are the contour of the barrel under the handguards which you can't see by looking at it, the LE6920 M4 carbine have the narrow area in front of the front sight which are primary used for the M203 grenade launcher of the US military, the barrel in front of the Expanse and the A3 Tactical carbine have no notch for the grenade launcher.
So the main difference are the contour under the handguards and also the markings at the left side magazine well of the lower receivers.
The LE6920 M4 carbine have a lightweight contour barrel under the handguards, it measures at 0.640" outer diameter, the lower receiver markings are "M4 Carbine CAL. 5.56MM".
Model CE1000 Expanse M4 is a new model for 2016 is a basic starter model below the normal models of other Colt carbines, it have a medium contour barrel under the handguards at 0.750" outer diameter but there are two lower receiver markings now, the first early markings are the same as the LE6920 M4 carbine (they're actually using LE6920 lower receivers) but now they're changing the markings to "M4 Carbine CAL. .223" also they're changing the prefix serial number (was LE) now to "CE".
The barrel of the Expanse is the same material 4150 steel alloy CMV (CroMoly Vanadium) as the LE6920 and AR6721 they're mil-spec grade steel alloy, but the Expanse have no chrome lining, the other two have chrome lining per mil-spec.
Expanse upper receiver is also different because it doesn't have dust cover and forward assist, also the upper rail have not T rail markings like "T2, T4, T6, etc" and no included rear sight, the dust cover and the forward assist could be added by the user later on if desired.
The handguards of the Expanse are small diameter old design with one layers of internal heat shields.
Expanse carbine buffer are standard carbine weight not marked "H" but the other two have heavy "H" marked buffers.
Last the AR672 A3 Tactical Carbine have a heavy contour we call HBAR under the handguards, at the rear half of the handguard it measures 0.950" and the front half are 0.845", the lower receiver markings are "AR-15A3 Tactical Carbine CAL. .223".
All of these three carbines have 5.56mm NATO chambers as marked on the barrel in front of the front sights, they also could safely fire commercial .223 ammo.
The handguards of both LE6920 M4 carbine and AR6721 A3 Tactical carbine are the M4 type oval section, large diameter with double layer internal heat shields.
One of these carbines are good for certain use, the LE6920 are probably the most versatile, the AR6721 are more for accuracy and mostly shooting stationary, you don't want to carry it for a long time because of its heavier weight, and the CE1000 Expanse are for beginners that don't want to spend much money to get into the AR platform but can upgrade it later, also for current Colt owners to add another pony to their collection.
Here are pictures of our carbines from left LE6920 M4, middle CE1000 Expanse M4 and right AR6721 A3 Tactical Carbine, note the left and right carbines are not factory configuration and had been upgraded.