Rule of thumb works like this:
Slots 1 & 3 have to match.
Slots 2 & 4 have to match.
Slots 1 & 3 have to be populated before 2 & 4.
Never mix ECC & non-ECC.
Never mix single sided & dual sided RAM.
If the stick have different speeds, lowest wins.
>3Gb can only be used by natively by a 64-bit motherboard, and a 64-bit OS.
What are the 4 sticks you're trying to use, and what motherboard are you using them in?