At age four, kids should be learning. But what matters is WHAT and HOW they're learning. They should be learning that learning itself is valuable, and that they're on the path to independence, at a pace that's right for them, and right for children in general. Learning to read will mean that they can go to the library and choose a book by themselves and read about what interests them (bugs, space, superheroes, what makes a truck go, how the princess went to the ball, the rules of certain sports). Learning to write will mean that they can make up their own stories and write them down, or write down a recipe for cookies, or write a letter to Grandma and receive a letter back. Learning about numbers means they can count the money in their bank, and understand about saving for something. But the proper steps in the proper order are essential.
And your son is missing out on some critical steps in the learning process. Would you expect a medical student to start out in the operating room with a scalpel, or would you hope that the student starts out learning basic anatomy, medical vocabulary, how to approach a patient who's in distress, and then progress on to increasingly difficult material in a logical prescribed order? Your son is being forced to jump ahead on some crucial learning processes.
At age 4, kids are learning how to function in a social and educational setting (how to share, how to listen, how to cooperate with other students). They learn shapes and colors, and a good teacher will be aware of potential problem areas. For example, if a 4 year old can't even recognize colors, or cannot sit down for a few minutes, or if he can't hold a crayon, or if he can't recognize his name on his desk or on a chart, the teacher might monitor that child and make sure that progress is made or get some extra help for that child. But being forced to "practice" something that he hasn't learned is detrimental.
Your child is operating on a frustrational level in writing. He finds it unpleasant, he has not mastered it, and he's not being instructed properly. Practice is for when a child has been properly taught. Your child might take piano lessons, for example, and be taught the scales, and how to hold his fingers, and how to play a scale. Then after that, he practices. He holds those fingers correctly, and he plays those scales until they become very familiar to him. He doesn't just get told "play scales until you get them right". After the frustration level, there is instructional level and then independent level. An effective teacher recognizes the levels and teaches to them.
Continually being forced to stay at the frustration level produces nothing but more frustration, and an increasing hatred for the subject, and low self-esteem. That's what your child's teacher is creating. She should recognize that he's not prepared to write yet, and she should be concentrating on making sure that he masters certain skills first, and then help him, at an appropriate age, to learn to write.
Get him out of that limited environment and recognize that he's telling you that he is frustrated and digging into a deeper hole. Don't stress. A 4 year old doesn't need to master printing or cursive. He needs to master gross motor skills, a love of learning, and an eagerness to try something new, at the proper speed and time.