OK, I tried with the bad sectors recovery, but it didn't change anything. Any image before 4.0 works fine, but 4.0 doesn't, it stays stuck at the "Discover" message.
P.S. My bootloader is currently 84 as it is on another DM8000 where OpenPLI 4.0 works fine. So I guess the flash is simply corrupt, but I have no idea how to reformat the flash...