I would suspect a bad power supply (not just weak, but real weak, gone bad), as David already said.
Next would be the mainboard.
If it is the power supply, well, good luck. They are very hard to come by and expensive if you do find one. The only other option would be a MOD power supply, which is better in the end anywise.
If it is the mainboard. Ouch. A whole 4000 will cost you less on eBay than to replace the Mainboard.
Northwest Tech sells a "replacement" power supply (not an OEM, and has a different on/off operation) and mainboards for the 4000. I do not know current prices, so check it yourself, but last I checked, the power supply was $129 and the mainboard was $225 (I could be wrong).
Or, check my post of about a week ago in this message thread (I have both a MOD power supply and a full 4000 with a MOD power supply I am parting with);
http://forums.procooling.com/vbb/sho...t=14191&page=2