Until now I have exclusively designed and built new stuff with V5.0.1 & 2, however I just spent a head-scratching hour or two working with PCB originally created under V4.0.7.
The sch file was OK but then I modified the circuit, and exported the net file. When I ran PCBNew Read Net list it threw up many errors, and eventually I realised that some subtle changes had occurred in the library naming from V4 - V5.
Little things like the library names had changed from Diodes_THT:XXXX to Diode_THT:XXXXXX; similary with Capacitors_THT to Capacitor_THT. and resistors. The “s” has been dropped from the library name.
“Housings” have been re-assigned to “Packages” and transistors have changed as well.
I am sure there is some logic behind this, but it makes hard work of translation from earlier versions.