I have updated the documentation for file format version 4. Additionally, there are a few more details in the docs for older file format versions, too, especially for shape attributes.
There are also new JSON Schema files for programatic validation:
- JSON Schema for version 4 .glyphs files
- JSON Schema for version 4 of the fontinfo.plist in .glyphspackage files
Note that Glyphs 4 creates new documents in file format version 3. Once you use a feature that is only representable in a version 4 file, Glyphs 4 will ask you whether you want to switch the file format version of the document to version 4. Confirm, and Glyphs upgrades the version for you. You can manually choose a file format in the Document tab of Font Info.