Because 4 is a number divisible by 2, you can get into a situation where it may skip all days in a month. For that reason, generally the ‘x’ in every ‘x’ day should be an odd number. For example, let’s say today is an even day, and I set the program to run from today and every 4 days from today, but on odd days only. The program is not going to run at all, because today and every 4 days from today are all even days.
You can change this by modifying the ‘starting in’ days. For example, in the above situation, if I set the program to run from tomorrow (starting in 1 day) and every 4 days from then on, then it will be able to run.