Rick And Morty S06e03 M4b ((link)) -

You're referring to the Rick and Morty episode guide for Season 6, Episode 3, also known as "The Rickshank Rickdemption" but more likely M4B refers to "Morty at Batch 74" but commonly abbreviated as M4B for Morty At batchB -Part B and A combined or S06E03 'The Rickshank Rickdemption Part 2’