Quick answer
Mr. Osomatsu (*Osomatsu-san*) Season 4 has an unaired 13th episode, "The Matsuno Ten," now streaming on Crunchyroll. It is a bonus installment beyond the broadcast run, available to stream as of this week.
Why a bonus episode matters here
Osomatsu-san thrives on standalone sketch comedy, so an extra episode is not filler, it is simply more of the format that works. Dropping it straight onto Crunchyroll, rather than a disc-only extra, is a reader-friendly move worth flagging.
Where to watch
Crunchyroll, now. "The Matsuno Ten" is available to stream there.
The franchise on the boards
A long-running comedy is a useful stress test for any ranking. You can see exactly where Osomatsu-san sits on SenpaiRanks' Consensus and Quality lists, then rate it yourself to weigh in. For more currently-streaming picks, our what to watch this summer guide is the shortlist.