the best way to join on just 4 characters is to do it in the SQL view.

If you want to do it in the query design view (the grid).  You can build a
query on the first table with the calculated field

Field: JoinThis: Left(SomeField,4)

Save that query
Now build a query on the second table doing the same thing

Now build a query with the two saved queries as the sources and join on the
two calculated fields.

Alternative
Build the query on the two tables and join on the fields that you need.
Switch to SQL view and find the clause

TableA INNER JOIN TableB ON TableA.Somefield =TableB.AnotherField

Edit that phrase to
TableA INNER JOIN TableB ON Left(TableA.Somefield,4)  =
Left(TableB.AnotherField,4)