You need to arrange for a conference with the teacher, your son, and yourself so that he clearly knows what is now expected of him as a fourth grader. Hopefully, the teacher will be able to give your son some ideas about ways he can learn to stay on-task better in the classroom. You need to ask the teacher if your child will need extra help to handle the work at this school.
Transferring schools is not easy at any grade level. Your son may be having problems making friends. Also, remember that many things are new to your son, and the adjustment could be affecting his grades. Once the teacher becomes more aware of the problems your son is having, the teacher should be able to ease his adjustment.
You can help your son with his organization skills. Work with him to organize his own daily schedule from the time he gets up in the morning, so he can budget time for study and planned after-school activities. Remember that all schedules need some built-in flexibility. Every minute of a fourth-grader's life doesn't have to be organized.
Establishing a routine for assembling all homework and things that must return to school the next day will help your son be organized. He should also be using an assignment notebook to keep track of all his assignments. A binder with separate subject sections or a folder for each subject will keep his work organized and make it easier to study for tests. Learning to be organized takes time. Give your son small increments of responsibility instead of expecting him to become organized overnight.