Yes, so she is younger than I suspected, and doing very well. At that age, and before the spectre of levels, grades, SATs etc gets a hold (of child and parent), most new learning experiences can be fun. And I am delighted you are sharing her enthusiasm!
In nursery once, a 4 yr old brought me a bit of moss, and asked what it was. I explained it was a plant, but didn't have ordinary flowers, and liked to live in damp, dark places. Before long I had 20 excited children searching for different samples of moss! (Ask some secondary pupils to find samples of moss, and they would tell you to take a jump!)
Re "Cinderella" - it is BRILLIANT that she can see the 'anomaly' [I had to check how to spell that!] of the 'C' sound, and get indignant over it. You can explain to her that letters and the sounds they make in words, have come from different countries, over hundreds of years, so they don't always make the sound you expect. Tell her, as she gets older she will find lots of words that seem 'silly' or 'wrong' because they seem to break some of the rules of letters, and they can be called EXCEPTIONS.
And tell her that numbers and counting is much more sensible, because they don't have many Exceptions: so she is FIVE (5), and 5 years, or 5 fingers, and 5 books - or 5 ANYTHING is still the same '5'.
In some children this initial enthusiasm isn't noticed or understood by parents (or even by professionals, sometimes) and it 'withers and dies' so the great potential is lost.
ENJOY her, and (for now at least) accept the challenges and 'moans' you might get from her. Although adults obviously KNOW more, young children can have a very special kind of insight and awareness, and function on planes that adults have long forgotten, or society and convention have eliminated from us.