OOPS! Post got cut off...updated...
Y-Y-Y-Y isn't a documented error code on the 4600 (page 2, here).
Is it only 2 beeps or is it a repeat of 2 beeps, 2-2-2 which would be a memory error.
You can try this, if you're comfortable working inside the case.
1. Power off and unplug
2. Press/hold power button for ~15 sec
3. Disconnect all perhiperals
4. Open case
5. Carefully re-seat RAM modules and PCI cards in their slots
6. Used canned air to clean out the dust bunnies
7. Remove the motherboard battery and press/hold power button for ~30 sec
8. Reinstall the battery (right-side-up!) - If battery is more than ~2-3 years old, this might be the time for new 3-volt CR2032 lithium ion cell battery
9. Close the case and see if it boots now with only mouse, monitor and keyboard connected