I have a link for v3.20 Enhanced which installs but i'm not able to update to v4.21
Normally a stand-alone product is only upgradeable until the next version number, i.e. 3.xxx to 4.xxx then 4.xxx to 5.xxx.
If you purchased 3.20, that wouldn't cover upgrading to version 4.21. However, sometimes, the upgrade is available if the purchase was made near the release of the next new version. If not, it's available at a reduced price.
You should contact sales with your purchase information as this seems to be an account issue, which can't be handled in the forums. They can assist you in getting the version you are entitled to.
As for reply #4, GameStop Now( Formerly Impulse Now) is a tray app update notifier. It has no effect on those annoying pop-ups by the systray when GS is launched, we are stuck with them as long as we use GS.