Use Arabic numerals for the volume

Warning²Federal courts, Illinois courts21st and 22nd editions

A volume that carries a number is cited in Arabic numerals, not the Roman numerals a work's own spine sometimes prints — "4 Yale L.J. 1057", not "IV Yale L.J. 1057".¹

1. 1R. 3.1(a) Volumes. Tables: T1 United States Jurisdictions; T6 Case Names and Institutional Authors in Citations; T13 Institutional Names in Periodical Titles.
2. 2A warning: the form is usually wrong under the rule in note 1, and context can justify it.