Design a digital Butterworth lowpass filter operating at a sampling rate of 80 ksamples per second with a 0.5-dB cutoff frequency at 4 kHz and a minimum stopband attenuation of 45 dB at 20 kHz using the bilnear transformation method.

%code
Wp = 0.1;
Ws = 0.5;

[n,Wn] = buttord(Wp,Ws,0.5,45)
[z,p,k] = butter(n,Wn);
[num,den]=zp2tf(z,p,k)
figure
freqz(num,den,80000)
[numd,dend]=bilinear(num,den,512)
figure
freqz(numd,dend)

After applying the bilinear transformation the plots obtained for analog and digital are different. Kindly help me in solving the issue.