The M4 engine is a private sampler at the moment.
You need certain requirements to be eligible to download it from Ameya's group. And from what I know, the invitations are currently closed.
I need to tell you that it's not that amazing. It WONT MAKE YOUR CV SOUND LIKE VCV as it was rumored. It makes the voices more breathy in some cases and smooths some transitions, but won't make them fluid as VCV or CVVC. So don't stress by the fact you can't get it at this moment.
TIPS, Fresamp, and tn_fdns are your best options right now.
EDIT: Also, this is on the incorrect forum... this should be posted on the Help sub-forum; so I ask and Admin/Moderator to move it.