Your server's motherboard probably only has 4 DIMM slots.
EasyDCIM.com - DataCenter Infrastructure Management - Reach Me: firstname.lastname@example.org Bandwidth Billing | Inventory & Asset Management | Server Control
Order Forms | Reboots | IPMI Control | IP Management | Reverse&Forward DNS | Rack Management
CNSERVERS.COM Portland, Oregon since 2005 Fully DDoS Protected BGP Network - Hardware Based Automatic Network and Application Level Protection
IP Transit / Colocation / Dedicated Server / Remote DDoS Protection via GRE Tunnel & Reverse Proxy
Portland, OR / Seattle, WA / Los Angeles, LA
Is this server leased or owned? If leased, your provider should be able to just make the swap for you. If owned, you made a bad initial investment getting 4gb sticks when you should have got 2 x 8gb sticks and just added 2 more when needed.