WHEN TO TEST??

So my cycles are really regular, usually about 29-30 days. So today is cycle day 28. But my app says I’ll start my period in 4 days (so the 19th) but I feel like that’s too far away... cuz by then my last cycle day will be 31... and I never go past 30 days in my cycle.. anyway my point is when is the EARLIEST i can test ?