You can either use the subreport approach where you link the subreport on the formula field, or you could create a command to use as your sole datasource where you build the formula into the linking in the from clause, as in:
select table1.field1, table1.field2, table2.field1
from table1
inner join table2 on
left(table1.acct,4) = table2.acct
-LB