What is an op-amp's differential input impedance?
The differential input impedance is R1 + R3.
That is because the op-amp actively makes sure via the feedback R2 that both V+ and V- nodes at the op-amp have identical voltages.
Therefore, as V1 terminates via R1 into voltage V-, and as V2 terminates via R3 into voltage V+, and as V+ and V- are identical voltages, there will be V1-V2 over the sum of resistors R1 and R3, and that is the differential impedance.
As an example, let's use the circuit to measure say a 3V battery. One battery terminal, say the positive, is connected to V1, and the other battery terminal, the negative, is connected to V2.
As the battery is a completely floating voltage supply, i.e. it shares no common reference with the supplies of the op-amp or the ground symbol, the measured battery voltage is completely differential.
So, V1-V2 is the battery voltage, 3V. Again, op-amp keeps V+ and V- equal, no matter what V+ and V- are. So there is definitely a 3V drop over resistors R1 + R3 only. Thus, what voltage there is at V+ is, and thus what is the current through R4, has no role in the differential impedance at all.