A-Tech 4GB Module for Lenovo ThinkPad T420 Laptop & Notebook Compatible DDR3/DDR3L PC3-12800 1600Mhz Memory Ram (ATMS350677B12034X1) at Amazon.com
How to Replace or Upgrade RAM Lenovo T420 - YouTube
Lenovo ThinkPad T420 Review: Kickin' It Old School - PC Perspective
8GB Stick for IBM Lenovo ThinkPad T Series T420 T420i T420s T520 T520i. SO-DIMM DDR3 Non-ECC PC3-10600 1333MHz RAM Memory. Genuine A-Tech Brand. at Amazon.com
CMS 16GB (2X8GB) DDR3 10600 1333MHZ Non ECC SODIMM Memory Ram Upgrade Compatible with Lenovo® Thinkpad T420 4177, 4178, 4180, 4236 Series - A13 at Amazon.com
Tutorial: How to Upgrade T410/T420/400 Thinkpad Laptop to 8GB - YouTube