Upgrading Codeigniter App

Hello, looking for some advice please. We took over a codigniter app from another developer, which I believe was last updated in 2015. The client has requested some changes to the app and also for us to look at future proofing it in line with their seperate website. I don't have experience with codeigniter but I see that version 4 is not backwards compatible but that there has been a recent update to version 3 - so my question is am I better to upgrade the existing app to the newest version of 3 or start with a fresh version of 4 and convert?
For more information on the project, it is a booking calendar with login facilities.
Any advice would be appreciated
many thanks