Master Gardener: What you need to know about fruit cocktail trees

Q: I planted this fruit cocktail tree six years ago.  It was supposed to have four fruits on it, and so far I have gotten only plums and peaches. The plums are great, but the peaches are small and fall off before they are ripe and the fruit inside is really thin and bitter. I water every two days and fertilize once a month. Is there any hope of getting good peaches on this tree? I have thrown away 100 bad peaches already!
A: I hear from quite a few people who have purchased 4-in-1 fruit tre