I am going along with Citradia. A 4 year old, and older, citrus tree should be fertilized 3 times equally spaced out over the growing year. The tree looks under fed in several elements such as nitrogen and probably potassium, with one standing out - magnesium. The problem now is that we are going into the winter months, a time of year when citrus are not fertilized. However, in the case of magnesium, winter would not deter you from apply a magnesium supplement. The common supplement normally used is Epsom Salts (Magnesium Sulfate). Epsom Salts can be applied either through the roots, or as a foliar spray. As a foliar spray, dissolve 1 tablespoon Magnesium sulfate in a gallon of WARM water PLUS 1 tsp. of a good surfactant and spray the upper and lower surfaces of the leaves. You might have to apply several applications several weeks apart. It can also be applied to the soil. Or you could apply one application to the soil covering the area of the trees drip line, then water it into the root zone, and several weeks later a foliar spray. If your area does not get a hard winter freeze you might also apply one fertilizer application of either a granular fertilizer such as a 6-6-6 or 8-8-8 or even a slow release fertilizer. NOTE: leaves already showing magnesium deficient symptoms (a green delta shaped leaf center with yellow out on the leaf edges) do not always correct themselves. However, when the deficiency is corrected the new growth should become fully green.