# How to trace out qubits from a multipartite density matrix [duplicate]

I have a density made up of 4 qubits. Say system A is made up of the first and second qubits while system B is made up of qubits 3 and 4. I want to trace out 2nd and 3rd qubits.

Is there any reference available to trace out qubits from a multipartite density matrix?