[cflsystems]

XC5 is here to stay - for as long as QT is in business and continues to develop it of course.
XC4 is left behind and it will phase itself out eventually. This process started few years back and it is supported by QT. While they kept some development for it over these years lets face it - QT has no interest of supporting 2 completely different platforms that do the same. Not to mention XC4 is harder to control from licensing point of view.

If you move to XC5 you have to invest in cart, modules, theme, etc. While with XC4 from 4.6.x to 4.7.x you may be able to keep most of it. However you may not be able to upgrade from 4.7.9 to future versions without actually breaking the store and all it has.

Few years back I would have advise you to just stick with XC4. That's no longer the case.
I don't know of any 3rd party deb that makes modules for XC4 anymore and most of them will even not support what they had outside custom coding. So unfortunately if you are looking to move forward XC5 is the choice or you can stay where you at with XC4.

Ideally (on paper) XC5 upgrades work without the need to modify anything anywhere. You may still need to test and make sure any 3rd party modules and custom coding is ready for the XC5 upgrade but usually between lower version changes all works out ok.

So far XC5 upgrades even between major releases are not what XC4 are. Moving from 4.3.x to 4.7.x allows for database upgrade only. Moving from 5.2.x to 5.3.x upgrades everything and if all your non-stock changes are compatible the upgrade is not as expensive as with XC4 one.

To me now it is not a hard decision anymore which one to go with - 1. stay with what you have - not worth the headache to upgrade XC4 anymore; 2. upgrade to XC5; 3. move to another platform. Is that simple.