Learn to Read at Starfall - teaching comprehension and phonics
is great, my 4 yr dd went from reading easy Bob books to reading early reader level 2 books thanks to it. Just read read read outloud alot everyday and make sure they have alot of beginning readers and Dr. Suess around at home.
If you want something you can try How to Teach Your Child To Read in 100 Easy Lessons, my dd hated it so we did not use it with her, but I taught another child to read from it and it was very easy. There is also the Explode the Code series.
But really she taught herself with Bob books at 3 and at 4 we discovered starfall.com and from there it has grown naturally. Just in case you are looking for a handwriting program, Handwriting without tears is GREAT! She loved the Leappad from 2-4 years but hasn't played it much lately.
We also play alot of language games and have a sight word board hanging in our school room.
Most importantly: Just make it fun, it is more important to have fun and build a love of books and reading at this age than to sit down and do formal book work!