Calculating symbolic input impedance of Multiple Feedback Low-Pass filters
Take it in stages. The input impedance is \$\frac{V_{IN}}{I_{IN}}\$
The input current is given by \$I_{IN}=\frac{V_{IN}-V_X}{R_1}\$
... where \$V_X\$ is the voltage at the junction of R1, R3, R4, C2
Using KCL, Vx is given by \$\frac{V_{IN}-V_X}{R_1}+\frac{V_{OUT}-V_X}{R_4}+\frac{0-V_X}{\frac{1}{sC_2}}+\frac{0-V_X}{R_3}=0\$
Use your transfer function to express Vout in terms of Vin and you're there ... eventually ... after some substitution!