Perhaps the following can help for solving step 4:
For the Butterworth as well as Chebyshev approximation there is a formula that gives you the required filter order n as a function of w (ripple), as (stopband attenuation, ws (stopband freqency).
These formulas contain the log function (for Butterworth) and the arcosh function (for Chebyshev). If desired, I can provide you with the formulas.
Inserting the actual n value (n=9) it should be possible to find the difference in stopband attenuation for both approximations at a certain stop band frequency.