Dell XPS 730X - Upgraded Memory only recognizing two sticks

I upgraded my desktop 730x to Crucial 4GB Single DDR3 1600 MT/s PC3-12800 CL11 Unbuffered UDIMM 240-Pin Desktop Memory Module CT51264BA160B and the computer is only recognizing two sticks.

I have tried reseating the memory sticks and no change.

Any suggestions?

Thanks,