I think you can use the nested if function if there are only 6 clients

like this

=IF('Client 1'!E4=SUMMARY!C7,'Client 1'!A1,IF('Client 2'!E4=SUMMARY!C7,'Client 2'!A1))
or you can add a master sheet linked to all the clients sheet, then the formula will be very easy. Use the Index (match()) formula