Based on https://en.wikipedia.org/wiki/Abstract_machine, I'm not sure this is what you want for AC5. M4 is really just the exposed interface for your library, so I would think the AC that would go with this would relate to your interface and what it does.
AC5: Change of underlying abstract machine.
Based on https://en.wikipedia.org/wiki/Abstract_machine, I'm not sure this is what you want for AC5. M4 is really just the exposed interface for your library, so I would think the AC that would go with this would relate to your interface and what it does.