A four-year-old Sweetheart on Gisela rootstock with healthy green foliage and good growth, no sign of disease, set hundreds of cherries which it then proceeded to shed before they reached pea size, retaining maybe a dozen. A nearby yellow plum does the same thing. Since the trees show no sign of distress, I assume it could be a lack of some particular nutrient. Or hours of sunlight perhaps? Any ideas out there?
Four years is much too young to support much fruit. Give it a few more years to build up energy reserves before expecting a good fruit set. You may also require other cherries nearby for suitable pollen supply to get good pollination.