I have been teaching public Pre-K and kindergarten for a total of 24 years and can tell you that you are doing so many of the right things. The reading to her, trips to the library, having her make up her own stories based on the pictures, etc. are GREAT things to do and very helpful. She has the desire and the motivation to read and that proves that you have done the right things to encourage her. Her attention span is normal for her age though. Most three or four year olds are not ready to sit and drill letters, sounds and words yet. Get some plastic, magnetic alphabet letters and let her play with them. Get her a placemat with the alphabet on it and play around with that informally when you are at the table. When you read to her point to the words from left to right as you read. Sing and recite a lot of nursery rhymes. Those are great for something called phonemic awareness, which has to happen before a child can learn to read. Talk about the pictures in the book as you read. Let her predict what is going to happen in the story before turning the page or getting to the last page. Let her try to predict what the story will be about before beginning to read. These things all help with comprehension later on. Talk a lot. The more vocabulary she has, the better reader she'll be. Encouage social dramatic play (house, store, doctor, etc.). Research has actually proven that children who have the best social, dramatic play skills by kindergarten have the highest reading and language skills test scores by the time they reach fourth grade. There are also lots of fun and exciting videos out there with famous cartoon characters like Clifford which help teach letter sounds and recognition, along with building phonemic awareness skills. There are great computer games out there too and they usually love playing those. Most important, have fun with her. I can tell you that when she is developmentally ready to read, she'll read and based on what you have said, it will not be a difficult task for her since you have done such a good job preparing her and encouraging her.
Nancy