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

It will be a lot of work to convert it to CI4. Converting it to CI3.1.9 should be easy. You can use the latest versions of PHP either way. Unless you need to rewrite most of the app stick with CI3.1.9.

Simpler is always better

@RmsGreig,

I agree with @donpwinston. A couple of questions... What version of CI is your code base in? What PHP version?

Thanks for your help, I will have a shot at converting to 3.1.9

It is currently on php version 5.6 and I'm not sure exactly what verssion the CI code base is, where would I find this?

@RmsGreig,

Look in the code here ... /system/core/Codeigniter.php
Look for a define statement like ... define('CI_VERSION', 'x.xx')