Episode 4 does not exist.
Here’s the accurate breakdown based on the most common DVD/streaming numbering: CB Strike - Season 1Eps4
Now more integrated into the agency, Robin's dedication to the job begins to strain her relationship with her fiancé, Matthew. Episode 4 does not exist
In a brilliant piece of forensic deduction unique to Episode 4, Strike realizes that the killer must have had "track changes" enabled on the manuscript of Bombyx Mori . In layman’s terms, only someone who had physically edited the document—not just read it—would know the exact layout of the murder scene described in the final chapter. CB Strike - Season 1Eps4