Here the 1858 Wright edition omitted the inflectional ending -st from the verb believe, probably unintentionally. See under 1 Nephi 14:8 for discussion of other cases, some in the earliest text, where the ending -(e)st is missing from verbs that have thou as the subject. The critical text will in each case follow the earliest textual sources in determining whether a verb should take the -(e)st ending when the subject is thou. Here in Alma 45:4, the 1874 RLDS edition restored the correct “believest thou”, probably by reference to the 1840 edition.
Summary: Maintain in Alma 45:4 the reading of the earliest textual sources, “believest thou”.