J.E.
I got my daughter the tag reading system for her 4th bday and she enjoyed using it but mostly for sounds and games ... she didn't really start using the reading part until she started Kindergarten this year. Since we read a lot as a family she did enjoy having her own books to read. In summary I think 4 was probably too young but I would've bought it eventually ...
From the above experience I bought a used junior tag reading system for my 2 year old on her bday and she absolutely loves it and is learning a lot. They both spend a lot of time reading their books together now.
Another option may be getting her the leapster. I did get that last year for my oldest daughter (she was 4 at the time) for xmas and thought she learned more from that than her books and I think she enjoyed it more.
Good Luck!