Calculating and plotting CMRR
Welcome to Electrical Engineering.
With such a circuit, you have a differential input (\$V_1 - V_2\$) and a differential output (\$V_3 - V_4\$). But you also have a common-mode input (\$\frac{V_1 + V_2}{2}\$) and a common-mode output (\$\frac{V_3 + V_4}{2}\$).
Then you can define the differential gain as \$\frac{V_3 - V_4}{V_1 - V_2}\$ and the common mode gain as \$\frac{V_3 + V_4}{V_1 + V_2}\$.
Then you get the CMRR of this circuit.
Hint: the common-mode gain is not 0.