To locate Mind Your Language Season 4 on the Internet Archive:
The remains the best, most democratic repository for this "work" of preservation. The episode files are rough-hewn, the metadata often incomplete, and the legal status fragile. But thanks to anonymous uploaders who spent hours converting old VHS tapes, you can still watch Mr. Bob Hughes struggle with the same basic verbs as his predecessor.
The Internet Archive hosts various community-contributed files that attempt to document and preserve what remains of this era.