Question: Is the DRM-free version of Quake 4 on GOG compatible with the Microsoft Windows XP operating system?
Answer: Yes, but be warned that version is unofficially supported. If you have technical issues with that version, GOG's support team reserves the right to not provide any support.
There's no noticable differences between the original and GOG versions of Quake 4.
The GOG Galaxy client isn't compatible with Windows XP.