Hi , I take care of a large orchard and it is normal for apple trees to abort some of its fruit , especially if it hasnt been sprayed for apple scab or other fungal or insect problems. I still think though that you should have had some apples remaining. Have you been in a drought like we have and do you have deer that might have eaten the apples? You have 4 year old trees , were they straight whips when you planted them or were they branched out with a straight stem and several lateral branches? To get good apples , you need to use a fungicide and insecticide that is registered for fruit tree use . You can buy an apple spray and read the label for when to treat . and water the trees during drought . You may just have to be patient and the trees will produce. I planted 4 trees at my house 2 years ago and i got fruit off of 3 of them but the one just aborted all the fruit . What kind of apples did you plant? JPM