By default, the database is located in this folder: C:\ProgramData\Hygiena\SureTrend and is called suretrend. It is a .sdf file.
You can verify where you are saving your data to in SureTrend 4 before backing it up by opening the software and clicking the Admin tab at the top of the program. Click Server Configuration Setting tab on the left. If your Config Type i set to SQLCe then you should see the location of your database in the Database Name box. Go to that location and backup that file before proceeding with any sort of SureTrend uninstallation or upgrade.