I have the exact same problem.
I have a brand new SR7005. I started the firmware update procedure -> Countdown started indicating 15 mins, then rebooted at 9 min. and then loop-reboot forever ('update-retry' - 'updating' - 'update-retry' - 'updating' .....) and then the 'Fail B4' message.
I did reset the network card with the procedure posted by Jsechler (many thx for the info). Al least it gets me out of the infernal loop of the unit trying to update forever, but just like Jsechler mentionned if after that I start back the firmware update procedure I then get the same result - failure!!!!
I also did the processor reset procedure but it does not change anything. One thing I noticed is that even after a processor reset + network reset if I do restart the firmware procedure then it indicates a time estimate of 9 minutes (not 15minutes as originally). So i wonder if there is some left-over corrupted garbage firmware stuff that stays in even after the resets - don't know how to do a complete full firmware reset that would bring me back to the original 15 min. countdown... it always start back at 9 min.???
So I'm stuck just like you guys... I did not contacted Marantz yet - please let me know if you find a way out of this...