XC 4 is easy to understand, edit & customize due to smarty.
The pain is there when it comes to upgrade, if your site is heavily modified.
If your site is not heavily modified & you don't want to upgrade it frequently then I will advise you to stick with XC4. As QT is going to support for next 2-3 years for sure unless some drastic decision comes.
XC 5's main goal is flawless upgrade feature but I guess it's still not working it that way for many users. But QT working on it & it future it's going to work more efficiently. But I still think that it's around 60% of XC4.
If new modules will stopped for XC4 & upgrade will stop releasing then we all need to move to XC5 or other solution.
I love XC4 & I am going to remain with XC4 till it alive due to it's simplicity. Even though I am using XC4 since last 15 years but it will take me much time to understand how XC5 works as it's totally differently coded than XC5 !
My advise is to stick with XC4 till you are getting upgrades/patches/support as I can see QT is still releasing important modules for XC4.
Best of luck
