If you go to crucial.com their memory finder lets you choose between the M4800 with 2 SODIMM's and the M4800 with 4 SODIMM's. So maybe you have the one with 2? In that case 2x8GB is the max. But be assured that that's enough for 99,9% of the users.
Did you use your Dell service tag to find the details of your laptop? Those should include the exact specs and all manuals.
Kees
Hello guys.
I am Dell Precision M4800 laptop. I have 4GB x 2 pre-installed memory modules on laptop and would like to upgrade to 4 x 8GB. According to the specification for thi laptop it has 4 DIMM slots. I have already found 2 slots right under the lid but cannot find another 2 slots. Could you give me a hint as to where the other two DIMM slots please.
Thank you.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic