Reversals can be quite common for children. I have found some children need to refer to their ABC line when they make certain letters through much of grade one and a few even through grade 2.
All children develop differently, and although reversals are a sign of dislexia, I would give her some more time to develop her writing skills. Just keep it fun while practicing with her! :)
(I also have a child with a disability so I understand your concern!)
Reversals are often seen into the later part of kindergarden (b,d,). A good list of symptoms in early childhood for dyslexia can be found at www.dyslexia.yale.edu. A list of fun ways to learn letters without holding a pencil is available at www.lynslines.wordpress.com
The short answer is........If there is no family history of dyslexia, try not to get too hyper. As a handwriting coordinator and author of The Mystery of the Land of Letters (a book to de mystify the writing process for young children) you should first practice what the words top, middle, and bottom mean. If your child starts writing from the top bottom/left to right (if she is a rightie - opposite for a leftie, the number of reversals will be reduced. Handwriting takes practice, skill, good fine motor strength, and an understanding of letter stroke sequencing. Before having you child write on paper have her write in the sky, in salt, sand, and pudding.
Reversals are VERY common, even into first grade. I think you should not be concerned at all! Just offer her the written or printed alphabet when she is writing so she has a visual of how the letters are formed or offer a friendly, gentle reminder so she doesn't become frustrated. She will receive direct instruction in K and 1st grade to correct this and the teacher will let you know if there is an issue of concern!
Probably, it is just her age. However, if it doesn't improve by first grade, you might want to do some intervention. Here are some suggestions. If she confuses b and d, draw a bed between b (the head of the bed, and d for the foot of the bed. Have her trace the b and d in dry jello, or make pudding and write in the pudding with her finger. Say the letter, as she writes. Cut out the letters she has trouble with, out of sand paper. Trace the letter with her finger. Tape the letters, with large writing on objects, such as d on her bedroom door, bed on her bed, large d on a doll, b on a ball.
There are also a log of websites for dyslexia that can give you ideas for other things that you might do. Right now, I would just go easy. Don't make her anxious that she has a problem. Just make it a game and spend just a few minutes. If she acts tired, or disinterested, let it go for the time being.
She is 4 years old! Developmentally she should not even be writing letters. You should be reading to her, talking with her, having her draw pictures and YOU writing words that she tells you about her pictures. She should be playing with clay, painting, coloring, toys that use her imagination (not batteries) listening to music, singing, dancing and not watching TV or videos. Let her be a child! Treasure every moment. Childhood is so very short!