Hello, and welcome to THG.
What #1 & #4 lights?
Dell's have diagnostic LED's on the motherboard (or some Opti's have them on the front panel).
1 and 4 being yellow with 2 and 3 being green shows to be a disk error. Try reseating all data cables to the hard drives, CD-ROM, and floppy (if present) and check all power connections to the drives. If this doesn't work, try disconnecting all drives and see if the computer will go past this point to a "Press F1 to retry boot, F2 to enter setup" error message or something similar to that.
1 and 4 green with the rest off shows to be a fatal system board error. Sometimes reseating all cards and memory will fix it. If not, remove all cards and add them back one at a time and see if one of the cards is hanging the system. If using a video card, remove it and try booting with the onboard video only, with no cards installed. Let us know if this helps.