The newer OMAp dual core devices no longer use SBF mode for flashing because they have fastboot support now, so RSD Lite was adapted to use XML.zip files that are an archive of signed fastboot images rather than an SBF, which is a Single Binary File consisting of CG images. The legacy bootloader is still available in the boot menu but you won't find the firmware files in SBF format for these devices.
That said, we are waiting for the full XML.zip for the D4 to be leaked and currently have no way to restore the stock factory signed images for this device.
This means it is very dangerous to do any significant mods on this phone at this stage.
Hashcode and others have done great work to make it as safe as possible, but the risk cannot be overstated.
If you render the device unbootable for any reason, your options are very limited.