We'd like to integrate our service with LDAP, but because of hardware constraints, we're only allow 4 digit user ids and passwords.

What would be the best practice for performing such an authetication?

We've considered adding User ID and PIN attributes to the LDAP user schema, but we're not sure how happy people would be with modifying their schema to interact with our service. The PIN attribute would have to have the same support that native user passwords have. (hashing and salting etc.)

UPDATE

Another consideration is how ldap_bind works with this scenario. How do we get it to use an alternate authentication method? Can this even be done without affecting other services that use the same LDAP server?