Q. My tomato bush is 4 feet high, why are the leaves turning yellow at the bottom and center of the bush?
A. It could be any one or a combination of several things.
1. What are you feeding your plants? lack of any of 4 different nutrients could cause some yellowing symptoms, including nitrogen, zinc, manganese, and iron. Those symptoms are somewhat different, so you need to learn the specifics of how each one affects the plant.
A great set of books to make you an expert on deficiency symptoms is The Garden Doctor 3-volume set, by Dr. J. R. Mittleider. They can be purchased on the website in paper, or for only $20 more you can have the entire Mittleider Gardening Library on CD ROM as a Searchable Database.