Assuming your field is called yearmonth and the table is sometable, start with this query:
SELECT Left([yearmonth],4) AS YearPart, Right([yearmonth],2) AS MonthPart
FROM sometable
Call it qryParseMonths and use it for this:
SELECT
IIf([MonthPart]<5,[YearPart]-1,[YearPart]) AS StartRange...