Rules to Enable Dual Channel Mode
Matched DIMM configuration in each channel
Same Density (128MB, 256MB, 512MB, etc.)
Matched in both Channel A and Channel B memory channels
Populate symmetrical sockets (Slot 0 or Slot 1)
Configurations that do not match the above conditions will revert to Single Channel mode.
The following conditions do not need to be met:
Same timing specifications
Same DDR speed
Memory channel speed is determined by the slowest DIMM module populated in the system.
*This really depends on your motherboard. Some manufacturers use a color-coded system (meaning use the same colors) and some require that you use the symmetrical approach (meaning same slot on both channels).
As for using four slots... are you able to get into BIOS? if so, try bumping up your voltage by ~.1; however, if you can't, test your RAM in pairs using MemTest86+ 4.1