Android 4.x is pretty old. Hence why sihui says FKB has ‘special older versions’ for 4.1 - 4.3. You could soon find FKB say 4.4 is an ‘older’ version and then you start to lose new features etc on updates.
If it is a fairly generic OEM tablet though you might have more of a chance to root and upgrade the OS. But I would try and find/pick one that has an identifiable model and then look it up on the likes of the XDA developer forums to find upgrade paths, although they can still have issues sometimes with full hardware support.
The HABPanelViewer also looks interesting as an FKB alternative but it’s Android 6 and up.