Poweredge 1900 Installing Memory 4*4GB

How can I benefit from my 4 GB memories on PowerEdge 1900?

I tried all the ways to install them and the only way the server was running when I put them in the last 4 slots.

But System memory still reading just 8 GB. And Windows server shows (16 GB installed / 8 GB usable).

I even tried to put one memory (4 GB) on slot 1, but server couldn't boot.

Is there any limitation in the first 4 slots?

Anyone could guide me to the best solution of installing the 4 memories with total of 16  GB?

regards,

Soubhi Attar