I have an orange tree for about 4 years. It bears flowers but they fall off. I have seen only 2 fruits from this tree. What may I be doing wrong. See pictures. Some leaves have holes. Not sure if that has anything to do with it.
They are getting pollinated well enough, you can see the young fruit hips in those pictures. If there wasn't any fruit set, the ovaries would fall of the same time as the flower and you'd see no hips. Your problem is either lack of or too much nutrition and water. Wide temp swings can also cause infant fruit to fall off.
If your tree had some oranges in the past and not now, but it has been growing normally, then likely you don't have enough pollinators, bees, in your garden. To compensate for that you can transfer pollen from the stamens to the pistils of another flower using a small brush. Repeat for all the flowers.
However, looking at your pictures, I see only pistils and no stamens so it looks already too late.
You've said your tree is 'about' four years old - most orange trees do not produce fruit till they are between 3-5 years old anyway. It's common for a large percentage of the flowers to just fall and not produce fruit, so other factors, such as the amount of sun, fertilization, and most importantly, water, become much more important in getting the tree to produce fruit, because there are other causes for flowers to drop prior to pollination/fertilization, such as dryness at the root or sudden cold temperatures when the flowers are present.
It is possible to pollinate by hand, should you wish to try it, and there's a video here showing how to do that
Orange tree flowers can be pollinated by hand, by insects such as bees, or by movement, either by gently shaking the tree yourself or the wind doing it on its own. Pollination of flowers on oranges is a little more complicated than it first appears - the variety of orange tree can make a difference, and there is some useful background information on the pollination requirements and process in citrus here
And some more general info here
The tree itself does not look as if it is in full health - check the backs of leaves and all the stems for signs of infestation by insects or other problems which may need treatment, and you may wish to use an appropriate feed/fertilizer to boost the plant's strength.