obtaining "4 Hz modulation energy"
The link in my comment suggests the following:


  I would recommend first extracting the envelope by using either a halfwave
  rectification (i.e., replace all the negative values in the time waveform
  with zeros) or a Hilbert Transform and then lowpass filtering the waveform
  at around 50 Hz (the lowpass is optional if you only care about the 4 Hz
  component).   Once you have the envelope, simply do an fft (making sure
  that your frequency resolution is at least 1 Hz) and look for the energy
  around 4 Hz.  If any of these steps don't seem obvious too you, let me know
  and I can send you some Matlab code.


So, what that means is you need to take your speech $s[n]$ and find the envelope:

$$
e[n] = \left | s[n] + j{\bf H}\left[s[n]\right] \right |
$$

where ${\bf H}$ is the Hilbert transform of your speech signal.

Then, take the FFT and look at the bin at 4Hz.