Slot 1 boots ok, slot 2 boots ok, slot 3 no boot, slot four no boot, slots 1&2 boots ok.
With slots 1&2 filled, Putting an DIMM in the 3rd slot no boot, 4th slot no boot, 3&4 no boot.
I've done this with each Dimm and get the same results.
Is that the Asus A8V-VM ? If so In the BIOS with the AI overclocker tune set to Manual you should be able to change the voltage under advanced / jumper free config / ddr ref voltage it is on pages 2-17 and 2-18 of your manual.