SatGen 3 and SatGen 4 are two separate software applications. They can, therefore, be installed simultaneously or independently and they will both work with any SatGen dongle you have or decide to purchase.
The dongle will work regardless of the SatGen version, so all you have to do to start using SatGen 4 is to Download and install SatGen 4 on your computer and connect the dongle before you open the software.
Note: SatGen 3 has now become a legacy poduct and will no longer be updated.
You can find the full User Guide for SatGen 4 here.