Z_eff for last electron of 4S sub shell electron of  Zn atom (Z = 30) is

Electronic configuration of Zn $\left(1{\mathrm{s}}^2\right)\left(2{\mathrm{s}}^{2}2{\mathrm{p}}^6\right)\left(3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}3{\mathrm{d}}^{10}\right)\left(4{\mathrm{s}}^2\right)$

For 4s sub shell

$\mathrm{\sigma }$ = 0.35 +18(0.85)+10(1)=25.65 

Z_eff = 30 -25.65 = 4.35

For 3d sub shell

$\mathrm{\sigma }$ = 9(0.35) + 18(1) = 21.15

Z_eff = 30 - 21.15 = 8.85