I cannot install Radiation Decay v.4 on Windows 10.
"Unable to save file:
Decay $.msi. The system cannot find the path specified."
This happens because you didn't run the installation as Administrator. I've tested both cases where I launched the application with and without Administrator access. I received the same error as you, but after I tried Run as Administrator, the software installed correctly.
I thank you from the bottom of my heart ! I'm shocked that solving the problem was so simple.
Best regards !