Hi!
Can you specify a bit more, please? In general, if you want to get the subsystem state, you will trace out the complement of it. That will result in the reduced density matrix (of 3 qubits in your case). It will not be a pure state but density matrix(which can be represented using an MPS with enalrged hilbert space/ancilla sites or using an MPO)
If you just want the reduced density matrix, discussion here might be useful.