Create a new table for the grouping.
Structure
tblYearGroup
-Year (PK)
-YearGroup
Data
Year | YearGroup
1990 | 1990-1994
1991 | 1990-1994
1992 | 1990-1994
1993 | 1990-1994
1994 | 1990-1994
1998 | 1998
Then create a query - add the 2 tables, join them by the year field, add YearGroup to the...