How random sample rows through many ranges of a matrix and delete from the original one and print in another one?

I generated a lot of outputs from a DFT rotine and put all of them in a single sheet and imported as a matrix to MATLAB, something like:

 U      n       m       erg
 0  0,0833  0,0833  -2
 0,050  0,08333  0,0833  -2
 0,100  0,0833  0,0833  -2
 0,150  0,0833  0,0833  -2
 ...
 6  0,0833  0,0833  -2
 ...
 20,00  0,0833  0,0833  -2

It goes from 0 to 20 for each test, so the whole matrix keeps repeating from 0 to 20 until I have almost 20k rows.

What I need is to random sample 40 rows from 0 to 6, then another 40 from 6 to 20, through the whole matrix. I want to able to sample and delete from the original one and print in another matrix.

Is it possible to automatize the whole process and/or is easier if I keep repeating the sampling manually?

Thank you.