Weird apple tree problem

I need some suggestions on one of my apple trees. It has not grown. I planted 4 trees 2 years ago all about 4' tall. three of them have grown drastically to as high as 16'. One though has barely grown 4 inches but is producing so many apples it is tough to keep upright. Seems like it is putting all of its energy into making fruit (which is great) but I fear all that weight is going to eventually snap the few branches it has. Other trees have an apple or two here and there that are kind of small but the little guy has fist sized ones already (about 3 dozen). Any help appreciated.

I would suspect that this one tree is root bound and thus feeling established and so is producing fruit, it also sounds like you need to thin the fruit to save the tree from overload, which will split the branches.

Fruit trees usually take two to three years from the date of planting to start producing fruit.
This is because they first settle in by sending out roots to gather the nutrients needed to produce fruit.
If a tree's roots circle around the original root ball, the tree will think it has established its new root system and this results in fruit set.
The problem can be high winds, for a tree that doesn't have a wide spread root system can be blown down far easier than one that has the wide spread base of roots.

I would 1. reduce the numbers of fruits per branch to a number that each branch can support well. 2. this winter, after the tree has gone dormant, dig it up to check the root ball and see the roots are circling. (I had this happen to a plum tree a couple of years ago and once I fixed the problem it has taken off)

Thanks, I will do that. Right now it is a charlie brown apple tree.

One time I had a honey locust with circling roots.  It was planted in pure clay.  After 4 years of minimal growth I took a flat spade and stabbed straight down where the edge of the original root ball was in line with the trunk (like a spoke on a wheel).  I worked my way around the tree and probably did 10 stabbings.  This cut the circling roots and gave them a place to venture out into the clay.  From then on it put on 4' per year.

If you want to try that I'd probably do it during the growing season or just before in the early spring so the cut roots can quickly take advantage of the cuts in the native soil to venture out.