I’m not sure whether this is a bug or whether the feature was intentionally removed. In Glyphs 4, it’s no longer possible to zoom in and out incrementally using Cmd+Space+mouse click and Cmd+Space+Alt+mouse click, respectively.
Those shortcuts should still work. Maybe macOS is interfering? Is it still working in Glyphs 3 for you?
Yes, it works in Glyphs 3.
Could you try relaunching Glyphs with plug-ins disabled (hold Option and Shift down while the app launches). Does that fix it?