Treat the money as lost or long delay issue and the loss was around Rs.4,000/-
Contact to the beneficiary (how ?) as you have informed that beneficiary is willing.
Get a letter from him to Bank stating that there are no transactions and the credit is not related, requesting his back to send the amount back to payee. Let him send such notice in a registered post.
Now collect the copy, a cheque for like amount and demand the bank to repatriate the amount back to send as it is a wrong transaction.
Send a registered notice to Bank enclosing copy of his cheque in your name, his cheque and his letter to bank and also an indemnity letter stating that you will reimburse the bank in case of future objection (enclose cheque in the name of the Bank for like amount, and on back of the cheque that is given in lieu of indemnity for wrong credit. The banker is safe as he is having a valid cheque of his customer, plus your cheque in case of objections from his audit supported with an indemnity letter from you.
If the bank fails make a complaint to Banking Ombudsman first.
So far your expenditure is just Rs.200/- for this exercise. Make an attempt first and let the bank state their objection in writing. Only if the bank refuses in writing, then only you can think of an alternate remedy depending on their objection/refusal.