You season 4 introduces a new character to the series, Rhys Montrose (Ed Speleers), who proves to be an exceptionally insightful character that Joe Goldberg (Penn Badgley) can’t help but obsess over. You season 4 introduces many new characters as the previous season sees Joe fake his own death and flee the country. Now he’s in London, posing as a university professor and living under the name, Jonathan Moore. While trying to adjust to his new life, Joe/Jonathan begins falling into the same dangerous routines he used to.

https://screenrant.com/rhys-montrose-you-season-4-hallucination-explained/