I wish I knew what to do, but in the mean time, here's some literature on how the lock works (it's for the milestone, but the d4 might use the same infrastructure).
The mbmloader: this loads the bootloader, if this is replaced with a version that doesn't check signatures, the bootloader can be permanently replaced:
The mbm (bootloader) does it's own signature check of the kernel before booting it.
If either the key burned into the phone's fuse, or the key the mbmloader uses to check the mbm are the same on both devices, one or both of those partitions can be flashed with with the unlocked version. If they're both different, this is a dead end.
The only other option after this (aside from espionage)would be to crack the signature system directly by either creating an unlocked version of the bootloader and patching it in a way that it generates the same hash, or discover a new way to factorize large (2048 bit) numbers, and reverse engineer motorola's private signing key. (If you were to discover this factoring method, nearly every security company would have to retool.)
edit: careful updating your phone, an OTA can relock your phone. The more I read, it seems less likely that the bootloader is encrypted. Dumps should be made, but this is going to require someone with greater knowledge than I.