A-Tech 8GB RAM for Lenovo THINKPAD T420 4177 | DDR3 1333MHz SODIMM PC3-10600 204-Pin Non-ECC Memory Upgrade Module at Amazon.com
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
A-Tech 8GB RAM for Lenovo THINKPAD T420 4177 | DDR3 1333MHz SODIMM PC3-10600 204-Pin Non-ECC Memory Upgrade Module at Amazon.com
Amazon.com: Lenovo ThinkPad T420 Business Laptop - Windows 10 Pro - Intel Core i5-2520, 256GB SSD, 8GB RAM, 14.0" HD (1366x768) Anti-Glare Display, ThinkLight Keyboard Light, DVD/CD-RW Drive, VGA : Electronics
Tutorial: How to Upgrade T410/T420/400 Thinkpad Laptop to 8GB - YouTube