the first descendant slot maketh module

First of all, a DDR3 laptop RAM module can’t physically fit into a DDR4 laptop RAM slot and vice versa. Notches on the connectors of memory sticks are on different places.

Sounds like you have either the wrong RAM module (spec) or the systems firmware is not up to date (I would be surprised if it's not already at the correct level). With the 4 GB module in place with the system working lets first try checking the firmware. Follow this Apple T/N: About EFI and SMC... - MacBook Pro 13" Unibody Mid 2012

Some RAM slots are color coded to reveal their slot type like DIMMA1, DIMMA2, DIMMB1, and DIMMB2. In the case of multiple RAMs, the first memory module is installed on the DIMMA2 slot to take full advantage of the multi-channel architecture.