It is great that you are trying to share the gift of reading with your boys from an early age. I am a preschool/kindergarten/1st grade teacher and I agree if another poster that suggested NOT teaching your son to read. I don't mean that you shouldn't share lots and lots of books with him and let him see you reading for yourself plenty too. At 4 he doesn't need structured reading instruction. This is the time to build a love for reading. Teach him that books hold wonderful information, fun stories and adventures and can be a nice time to snuggle up with mom and be close. You need to motovate him to be excited about learning to read. If he takes the lead and asks questions about words and wants to know what sounds letters make or what a particular word is, answer him and be open to his requests for knowledge, but you don't want him to learn to associate books with frustration and struggles when he's only 4.
My daughter just turned 3. She loves to find "her" letter - first letter of name - in books, on signs, etc. She recognizes her whole name now too and is excited when she finds it. She likes to play with magnetic letters and puzzles. She knows some of the names and likes to sing the ABC song. She matches the other letters in the alphabet to other important people in her life. So she associates the letters with words that are meaningful to her. This is really enough.
As a teacher of children first entering school, I saw many parents that felt like their children needed to begin school days, weeks, or months earlier than the age deadline. They all argued that their child could read and do math problems and such wonderful things. What we found in the classroom was that these children usually had difficulties sharing, taking turns, participating, following directions, and accepting disclipline. When entering preschool or kindergarten, foundations for these skills are so much more important than knowing "academic" skills before entering school. I don't know if you feel you are helping your son to "get ahead" by learning some reading skills before entering school or if there is another reason why you feel this is important right now. Typically by the end of 1st grade the kids really all catch up to their peers so the children that might have hada slight "advantage" by reading before school, no longer stand out in that area.
Help your son learn some basic self-help skills and help him to learn to be curious by exploring the world with him. Crawl around in the grass and look at leaves and bugs, read books together while snuggling under a cozy blanket, go to the museum, fingerpaint with your son, and help him to be excited about trying new things and help him to want to learn more. All of these things will do more for him than getting him to put together letter sounds. He will learn plenty about letters and sounds and how to put them together beginning in kindergarten and it is a primary focus in 1st grade.
Be cautious not to stress your son out as that can cause learning difficulties through the school years. It's just not important that he know how to read right now. I wish you luck in making choices to help your son to develop a love of learning for his entire life.