Upgrade to CI4 while running CI3

Hello,

I searched the forums for this, hoping someone perhaps had the same thought but didn't have any luck.

I have a decent size SaaS application built using CI3; it's update to date already. I would love to switch it to CI4 entirely, but that's a rather large task to take care overnight.

I am considering adding new functions (for example a stock manager, blog, a report that downloads as CSV etc.) using CI4 and then slowly, convert the CI3 portions to CI4 - one function at a time almost.

The SaaS deals with member sign up, login (so sessions) that would have to be shared between the two versions so users can move between them seamlessly.

The current application is running on Apache 2.4 and PHP 7.4.

I suppose this is mostly wishful thinking but would love any feedback.