I have never tried to expand the memory, so I can only quote from the manual: The Qnap TS-464 can be equipped with a maximum of 8G Ram. You can buy the device with 4G or 8G RAM. Both RAM modules must be identical, so max 2 x 4GB !
The manual can be downloaded from https://download.qnap.com/TechnicalD...
and the information is on pages 3 and 4. On pages 35 to 40 are also the instructions for the change !
Conclusion: Only with the TS-464-4G can the 2 x 2GB RAM modules be replaced with 2 x 4GB modules.

the -8G model already has 8GB Ram installed and cannot be expanded