I just compared a VF exported by Glyphs 3 to one from Glyphs 4 (using the same source file).
I did a quick side-by-side comparison in https://wakamaifondue.com/ and the only difference I could spot is that the font generated by Glyphs 4 does not seems to write the names for the stylistic sets (at least not in a way that wakamaifondue can read).
Is this a known problem?