The bank clearing number a.k.a. Bankleitzahl (BLZ) is a way to identify the location/branch of the bank.
In the IBAN, this is represented by a 5-digit number from position 5-9. Banks here usually require and quote only the last 4 (out of the 5) digits. If I'm not wrong, domestic bank account numbers also start with the same 4 digits.
If you know the location of the branch, you could search for the BLZ with this tool, or vice versa. http://www.banksuche.com/ch/