Although I am only a student; I have a seven year old sister. I am away for college but I always tell my mom to limit the amount of TV she can watch, purchase workbooks and computer games that enhances the ability one's ability to read. While everything I just mentioned may be tools that you have already tried, the most important is to read with your children. Read to them, let them read to you, and play reading games. It makes a big difference when you are actively involved in your child's learning process. Games are always helpful strategies that encourage interactive learning.
My sister read but she dint like to read often; but, I made her read something everyday! It sounds bad to force but sometimes you have to be strict when it comes to things such as reading.
Four year olds have very very limited attention span. Is there a way to make the homework fun such as using the letter he is working on with his favorite toys? For example if he loves baseball and is working on the letter A. A could stand for Astros baseball team. You could help him print a large A on a piece of paper that would then be taped or clothespinned to his shirt. He would become an Astros baseball player while you and him played ball. You could practice counting and writing the numbers every time he hit or caught the ball. For more fun activities you might look at the book Alphabet Soup: Stirring Your Child's Interest in Letters at www.lynaot.com. There is also a video demonstrating a fun way to play Air Hockey Baseball at that site.
I right there with you! I have a four year old as well that brings home reading home work each day. I try to make it as fun as possible for her. You could try letting your son create a special place to read. For example, you could set up a small tent in your living room and read by flashlight. One day you could have a reading "picnic" on a blanket outside. Visit the library and let him read to you there. Libraries in our area have a program called Doggie Tales, where they bring in dogs for the children to read to. Discover what your child's interests are and find ways to implement reading within those interests. Good luck!