My aunt learned to read at 4 while looking at the book as my grandfather read her stories. They later found she had a genius-level IQ. She used to read calculus books for enjoyment!
Our #2 son learned to read about that age, and in the same way. When he moved from his crib to a bed, I'd lay down with him and read until he fell asleep. I'd point to the words as I read them, and before I knew it, he was reading all on his own.
On a family trip, a few months before he turned 4, we were traveling with hubby's sister and b-i-l, and trading drivers as we rode through the night. At one point, b-i-l was driving our car while hubby was driving the little bus his sister's family was using. I slept in the back seat of our car, and #2 son was in his booster seat in the front seat with my b-i-l. A few hours later, when we stopped for breakfast, b-i-l asked, "Did you know David could read?" I said sure, and asked how he knew. He said that David read all the highway signs for him as he drove down I-81! ;o)