I see other Va soldiers misidentified in Mo Archives records as 8th Mo Inf, eg James M. Ables:

In National Archives records, they are listed both in 4th Va Inf and 8th Mo Inf, with same rank!

The Hugh Cain from Stone Co, Mo is probably one who enlisted in 72nd Rgt E.M.M. at Springfield, Mo 25 Nov 1862 and discharged for disability 18 Feb 1863.

http://www.sos.mo.gov/TIF2PDFConsumer/DispPDF.aspx?fTiff=/archives/AdjutantGeneral/Civil_War/ServiceCards/s807/1286.tif&Fln=S68674.pdf