How do I create a 4-on/4-off shift pattern?

With a 4-on-4-off shift pattern, the whole shift pattern (and therefore each shift within it) will repeat every 8 days.  

So, on your ShiftApp schedule, create your first 8 days of shifts, that is up until the point the shift pattern is about to start repeating. 

Then set each of these shifts to repeat every 8 days.

Now your four-on-four-off schedule is complete!