1. IF the 4th floor is really illegal, THEN you would need to get a stay order on the notice, till you complete your investigations.
2. Banks give loan against mortgage of property and based on Title-Ownership and has usually got nothing to do with unauthorized constructions, which the Bank hardly checks since they are more interested in business.
3. Procure the sanctioned building plans from the Municipal Corporation /Town planning dept., /Builder /earlier owner and check the building status.
4. IF the 4th floor is really illegal, THEN you would need to get it regularized (alongwith other flat owners) by hiring the services of an registered architect.