Well I don't know for sure, but I think your license is good for minor versions of the program.. For example if you purchase a license for 4.01 it should work for 4.02 and 4.03 but I don't think there is any guarantee it would work for major later releases such as 4.8 and 5.0.
PS: There is no way to download older versions from the website, you may have to contact NCH about that and they may charge a fee for providing it to you (not sure). Always keep backups of the installer or order the backup CD next time.