CMS 4GB (1X4GB) DDR3 12800 1600MHz Non ECC SODIMM Memory Ram Upgrade Compatible with HP/Compaq® Elitebook 840 G1 - A25 at Amazon.com
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HP EliteBook 840G1 Laptop Computer, 1.60 GHz Intel i5 Dual Core Gen 4, 16GB DDR3 RAM, 500GB Hard Disk Drive (HDD) SATA Hard Drive, Windows 10 Home 64Bit, 14" Screen (Used) - Walmart.com
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HP Elitebook 840 G1 RAM Replacement - iFixit Repair Guide
4GB Memory Upgrade for HP EliteBook 840 G1 DDR3L 1600MHz PC3L-12800 SODIMM RAM (PARTS-QUICK Brand) at Amazon.com