Edward Granville Browne
It has been stated that Mírzá Yaḥyá was the fourth of the Letters of the Living (where the Báb would be the first) by Edward Granville Browne in A Traveller's Narrative (page xvi). The book does not include any other details of the Letters and is against the Baháʼís' commonly accepted view that Mulla Ḥusayn's brother and nephew recognised the Báb shortly after him (since they'd take the third and fourth place).