My XPS 720 won't complete boot and displays the following lights on the front: 1, 2, and 3. Light 4 is not lit. The manual does not show what this means. I can get into the BIOS and it recognizes that the cdrom and two SATA drives are in their respective slots and apparently working satisfactorily. This computer has 4 gigs of RAM.
Undocumented light sequences normally indicate some type of motherboard issue. But it may also be a componet. So strip down the system remove all the ram sticks, remove all the cards, unplug the power to all the drives.
Boot the system and see were there lights shake out. if you get a good code start adding things back in one at a time.