It's pretty but once she starts school she will have a lot of exposure to letters, all day at school, and a big push for letters, reading and writing. I wouldn't cover a whole wall of her bedroom too, especially in capitals only as she'll probably learn lower case first. I think a 4 or 5 year old would rather come home to pictures. If you want to do letters, paint a plain wall and use stickers, so you include the lower case and it's less "all over".
Random other suggestion: this owl one
My 9 year old has a passion for purple. I found a medium, bright purple from valspar (too bright for mainstream dulux etc!) and she has that on one wall and a much paler, washed out version on the others. Her bright purple wall is about 3 years old now and she still adores it. It's a short wall of quite a rectangular room, opposite the window, so it doesn't look like a traditional feature wall or dominate IMO (but I would say that!)