As you are an educator, you surely know that a 4-year-old shouldn't be expected to write her letters straight, assuming you mean that she can't keep on a straight line. Indeed, she shouldn't even be able to write her name at all, straight or crooked or slanting. It may be that her pre-k teacher is urging such skill development, but I can't believe that any well-trained early childhood educator is concerned that your daughter doesn't keep on a straight line. If so, she is not helping the children in her classroom. Rather she might be increasing the likelihood that those children will resist formal academic instruction a couple of years down the road. Turning little children off by pushing literacy too hard is almost as big a hazard as not providing enough stimulation and instruction during the early years of life.
The first thing I would recommend is that you schedule a meeting with her teacher and discuss with her your daughter's general performance, socially as well as academically. As an educator, you know how critical it is that there be close communication between teachers and parents. The second recommendation I would offer is simply to relax and enjoy your daughter. She is bright and learns quickly, and this is a great blessing. Read to her daily, talk to her, play with her and savor the joy in such activities. Before you know it, she'll be writing straight and won't care whether or not you read to her or play with her.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.