The short answer is no, they are not. There really is no such thing as an M15A4.
The classic M15 is a squad automatic weapon variant of the M14 rifle. The catch here is that neither the M14 or the M15 has a split receiver, it’s a single piece.
So in other words, it’s not compatible at all with an AR-15.
If you’re looking for compatibility, the closest you may get is the following list of Ar-15 and/or Ar-10 parts:
- Buttstock assemblies
- Triggers and trigger springs
- Bolt catch roll pin (Armalite Style only)
- Hammers and hammer springs
- Pistol grips (a spacer may be needed for certain models)
- Receiver extension (Buffer Tube)
- Disconnector and disconnector spring
- Buffer detent
- Magazine release buttons and springs
- Safety selectors