My son is a VERY good reader (6yo 1st grader, reading approximately 2-3rd grade level) but I never did any formal reading program (or any program really!)
Just read to them all the time! What I think made the biggest impact on turning my son into a reader was first, reading together OFTEN, then later when we read together, I'd help him recognize a word (ie, ZOO which is fairly unique and easy to recognize) and when we read, for example: Put Me In The Zoo, the word zoo appears regularly throughout the book, so I'd read and point to the words as I read (which I had been doing for a little while already) then make a game out of it or share the reading. "Zoo" was his word to read, I would read all of the other words. When I came to "Zoo" my finger would stop and wait for him to say it. "Max" in a couple storybooks we had, etc. The goal here is to pick out easily recognizable patterns that appear often enough in the book to make it fun. We also did "dog" in Go Dog, Go! but 'dog' blends in and little eyes sometimes have trouble with d/b, etc, so it is not a good word to start with.
You'll learn to pick "easy" words that are reoccurring in the books you read to him. We've used "Little Red Riding Hood" as it was all capitalized and predictable and easy to pick out from the rest of the text, and boy, is he proud when he reads that long name! We didnt start with this though, we worked our way up to this.
Do it as long as it is fun, don't push. It is not that fun for my son to be the only reader in his reading group. But since we are deaf, my family all likes to read and it is an important way for us to get our information since talking with people is hard. (Not all deaf people like to read tho, it is hard since they did not have accessible language when they were young.)
Sounds like your 4yo is ready to start this fun game. I would not bother "teaching reading" (as in a program, etc) to the 18m old, just reading with her *is* teaching her reading. Pointing out or reading out loud things that you read throughout the day, saying what words you're looking for (for example, "where is PLAY on here, oh I found it, PLAY(click)" when you're using the remote, or whatever it is you use. So they learn that words are tools in your environment and pretty soon, maybe too soon, they'll learn how to start the TV and DVD/VCR player all on their own! Point out STOP signs, other signs when you're out walking. Just talk talk talk about their world, and read, read, read - to and with them, as well as for yourself.
And dont ignore math! Do the same with numbers and math concepts. Think & count out loud how many plates you need to set, things like that.
Sorry this is already long, but I read Elizabeth M's comments down below, and I think she is right-on. My son is bi-lingual with Sign Language and English, and I do think it makes a difference. It did not put him ahead *right* at the age kids begin to talk (maybe because he is a boy, as two hearing neighbor girls the same age were signing before he was!), but he is definitely way ahead now. Check out Signing Time for an easy and oh-so-useful "foreign" language to do with your kids. Signing Time DVD's are available in the Maricopa County Library system - they are fun for kids and very effective for adults as well. Also, I cannot say from experience that reading programs don't work, but I *can* say that simply reading to your child DOES work!
Relax, have fun, read, go to the library every week or two, cuddle up and read them! Enjoy it yourself, and then he will too.