[Rambo1984]

What are the primary differences between an AR15 and a M4A1?

 

[Bidah]

Well, which AR15 would help. Mainly the semi-auto and Full auto capability.

Outside of having the rifle be an SBR and paying the tax, the AR15 would be no shorter than 16 inches, and a real M4 is usually 14 inches, or sometimes 11.5 inches.

That is the baiscs. The M4 has a particular barrel profile, but you can order that as well for you AR15.

 

[Etho]

Military wise:

M16: 3 round burst, 20" barrel, fixed A2 stock, different barrel profile.
M4: full auto, 14.5" barrel, collapsible stock, M4 barrel profile for the attachment of an M203.

Civilian world nothing other than barrel length, gas system and stock.

 

[RMTactical]

An M4A1 is an AR15, but an AR15 may or may not be an M4A1.

 

[AR Hammer]

"An M4A1 is an AR15, but an AR15 may or may not be an M4A1."

Only in the broadest sense...
I sure don't include an M-4 as an AR-15...

AR-15 is a semi-automatic civilian rifle that was occasionally used by the military in the mid 60's.
The AR-15 was designed to fire the .223 Remington round.

An M-4 is a purpose designed & built military weapon.
It is fully automatic. It has fundamental design changes away from the AR-15 format.
It fires the Military/NATO 5.56x45mm round
(and shouldn't be confused with the civilian .223 Remington round).

The M-4 shares less with it's design parent, the AR-15, than any variant to date, and should not be confused with the old standard AR-15.