Hello!
I'm sorry it has taken me so long to respond. I thought I had set all forums to notify me of new posts, but apparently I had not.
I have several suggestions.
First -- If you want him to "someday" sit or play quietly while you read, then you want to begin training toward that now. But stay within his abilities. Choose some reading material that can be finished in 5 minutes. Have him participate in THAT part of the read-aloud time. Then set him free or use the DVD. After 4 or 5 weeks of doing this on a regular basis, you should be able to gradually increase the time. When you reach his limit, you will know it. Don't take his behavior personally -- take it as a signal.
Second -- Read material that engages him. And be engaging in the way you read it! Make the story come alive and he will stay with it longer. We love A.A. Milne's Pooh stories -- even a 2-year-old will sit still for those. Mother Goose is another good one -- catchy little rhymes that you can repeat when he's NOT sitting still.
Third -- and most important... He is only 4! If you push him to be too much a part of the learning process at this stage, by the time he is 6 he will hate school and everything like it. Set sound/movement boundaries -- "Sweetie, brother and I are doing school right now. You need to play more quietly. After we finish math, you and I will play a game. How does that sound?" You certainly want to train him to be respectful of the school going on around him -- but don't expect him to be able to participate in any meaningful way. If you are playing a game with older brother that he wants to join, find a way for him to join. If he comes around to look over your shoulder as you are looking at a science reader, scoop him up onto your lap and let him see. Then when he pushes away, let him go! He will give you very clear signals about what he can handle at this stage. Don't push for more!
I keep a cabinet full of manipulatives, counters, pattern blocks, dominoes, puzzles, etc. close to where I do school with the older children. The toddlers and preschoolers may play with those items while we are in school, as long as they stay within eyeshot. It makes them feel like they are "doing" school. AND it is building some readiness skills. I do NOT work with them on these items except occasionally in a very playful way. I follow their lead completely. I never push a 4 year old to do "five more minutes" or "one more page". When they are done they are done and that's all there is to it.
Hope this helps!