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: [https://support.apple.com/en-us/HT201518|About EFI and SMC firmware updates for Intel-based Mac computers]. Make sure its at the listed level or newer.
If the firmware is OK, give us the part number of the Corsair you ordered and take a good image of the labels on one of the modules as well as the memory chips so we can read them and post them here as an answer.