Your laptop will support up to 2 8GB modules for a total of 16GB RAM of the supported type memory. Overdrive might be showing 4 dimm slots but there are only 2 according to Lenovo's manual for that unit. If you want 16 GB you will need two new 8GB modules and you will not be able to reuse the 4GB modules you currently have. Unless you are running high level graphics, CAD, video encoding or other specialty applications it's doubtful you will ever use anything beyond the 8GB you already have, and probably not that. In any case, if you want to upgrade to 16GB that's what you'll need to do.