I am assuming you mean 4 sticks of high speed RAM (DDR3-1333 or higher, or DDR2-1066)
If so, it is a combination of all of them

The CPU is the biggest factor, but the BIOS (mobo) has to allow them to run at full speed. I am running a Phenom II X4 955 BE on a gigabyte GA-MA785G-UD3H with 8 GB of PNY XLR8 DDR2-1066. The system runs flawless. Another machine with an athlon II X4 630 had 8 GB of the same PNY DDR2-1066 and also ran great (GA-MA78GM-US2H) Curiously, a twin system of that machine wouldn't even boot with 8 GB at ddr2-1066... I am assuming earlier stepping of the cPU.
If you run them at lower speed (DDR2-800 for example) all the combinations of mobo/cpu and ram will be rock solid.
Alex