Generate the transfer function of a low-pass fourth-order Butterworth filter for cut-off frequency of 1 rad/sec.

Create the rffilter object.

filtobj = rffilter('FilterType',"Butterworth",'ResponseType','Lowpass', ...
    'FilterOrder',4,'Implementation',"Transfer function",               ...
    "PassbandFrequency",1/(2*pi),"Zin",50,"Zout",50);

Use tf function to generate the transfer function.

[numerator,denominator] = tf(filtobj);

The numerator of the transfer function for S21 is:

numerator{2,1}

ans = 1×5

     0     0     0     0     1

The denominator of the transfer function for S21 is:

denominator

denominator = 1×5

    1.0000    2.6131    3.4142    2.6131    1.0000

The corresponding polynomials in factored form is:

filtobj.DesignData.Numerator21

ans = 2×3

     0     0     1
     0     0     1

filtobj.DesignData.Denominator

ans = 2×3

    1.0000    0.7654    1.0000
    1.0000    1.8478    1.0000