Hi xpez2000,
On a Windows computer here's how I have multiple versions of Terragen installed.
After installing a version, make a copy of the Terragen installation folder and rename it something like Terragen 4 - 4.5.31. Now you have a copy of that version of Terragen you can run anytime.
The version of Terragen in the Terragen 4 folder remains the most recent install.
With that said, you may need to uninstall Terragen, then install your oldest version, i.e. 4.0 and repeat the above process until you're back to Terragen 4.6.31.