Hello Viktor Usov,
What error your are facing. Please elaborate. For running Optisystem there is not much need of RAM. Even you can run Optisystem 14 on 2 GB computer. If your system has 8 GB RAM, This is more than Sufficient. I think your error is not with the RAM as I also upgraded recently RAM to 8 GB. Need more explanation from your side if possible screenshot to resolve the issue. Thanking you.
Regards,
Dhiman