cceps (complex cepstrum) function in Matlab
You should split the signal up into overlapping chunks of 20 to 30 ms. It is certainly valid to take the complex cepstrum on the 4 second signal. But, since the frequency characteristics of speech are constantly varying the complex cepstrum on the 4 second signal will average all of these together and you won't know what happened when. If you take it in each 20 ms window then you can see how the properties of the signal change over time.