Below, I show 2 ways to extract every 4th line: lines 4 and lines 8 in somefile.

1. sed
   

   $ sed -n '0~4p' somefile
   line 4
   line 8
   $

   
   0~4 means select every 4th line, beginning at line 0.
   
   Line 0 has nothing, so the first printed line is line 4.
   
   -n means only explicitly printed lines are included in output.