No, you can't use the 6.3.2 uninstaller to uninstall 6.4.0.

If you can't find the uninstaller for M8 (which means there is a bug in the M8 installer), you can manually delete the "
QNX640" directory, delete the "C:\Program Files\QNX Software Systems\qconfig\*6.4.0.xml" file, and delete all lines 
containing the string "6.4.0" in a file called "C:\Windows\vpd.properties". Don't delete that file because you need it 
for your 6.3.2.

You don't need to delete your QNX* environment variables because you are going to re-install the release version of 6.4.
0. Environment variables have not changed for 6.4.0.

There is a Win32 utility to help doing all the above works, but I don't remember where that utility is.