You may check this guide
https://docs.appthemes.com/classipre...classipress-3/ first on updating from Classipress 3.xx to 4.xx.
It is recommended that you test that upgrade first using a clone test site of your install. In this way, your live site is not affected when something is not working right during the upgrade process. Once the update works on the test clone site, you can apply the same process to the live production site.
As always, backup your database just to be on the safe side.
- Jomark