Memory not being used

Hi

I put 4 x 2GB of ram into my xps 2100, and that appeared to work fine.

However since then I had to do a full windows reinstall, and now the memory isn't showing.

Ran a memtest and it's saying 4GB, same as the motherboard at boot, however cpu0z says 8gb

It's a 0G3HR7 motherboard.

any advice/suggestions?

Thanks