Installer Archives: GOG allows you to download "Offline Backup Installers." This means you can keep the 1.4.3.6 setup file on a thumb drive and install it on any of your personal machines forever.
"Elias. You dug too deep last time. Remember the hellevator? Remember what you found below the Underworld? The game remembers you. Launch it. But don't build any houses on the eastern hill. Not again." terraria 1.4.3.6 gog download
Once you have the setup_terraria_1.4.3.6_(64bit)_(12345).exe (or similar naming scheme): Installer Archives: GOG allows you to download "Offline