Chabad.org — English translation + Rashi
Torah Temimah points to Yerushalmi Horayos 2:1.
Sefaria — Yerushalmi Horayos 2:1
English: The anointed [High Priest] is like an individual: just as an individual, if he ate by the ruling of the beit din, is exempt, so too the anointed kohen.
Comparison of Mashiach to yachid — the anointed High Priest is treated like a private individual when the sin follows a beit-din ruling.
A High Priest who follows an erroneous beit-din ruling is exempt like a private individual. The Yerushalmi equates the anointed High Priest with an ordinary person for purposes of exemption when the sin was committed under the authority of the court. This prevents the High Priest from being treated more stringently than a layperson in cases of collective judicial error and keeps the special bull of 4:3 limited to independent sins of the High Priest himself.