Downloaded and compiled r4-beta2 on my anemic little RPi W tonight. Seems to be working fine as usual though for some reason, I still see the Qt Serial Port backend option, though I did *NOT* see it in the r4-beta1 build I did yesterday. Weird. Not sure I goofed up a git terminal command when I cloned the r4 repository tonight or what. The About page shows I definitely built Beta 2 though.
maybe delete the whole thing and start from scratch, the files for the Qt serial port backend aren't in r4-beta2 here, and it doesn't show up on linux or windows.