We can split the differential amplifier in two halves. The upper with R1 and R2 and the lower with R3 and R4, then sum the results.
The upper part can be drawn with an op-amp and then analyzed by means of the algebra of block diagrams:
as you can see, if A --> inf then Gain=-R2/R1
H and K comes from the superposition (apply Vi and short Vo to calculate H, then apply Vo and short Vi to calculate K).