This possibility should be explored with your son and his teacher. Armed with information from your son, make an appointment to meet with your child's teacher as soon as possible. Start the conference off on a positive note, such as thanking her for making you aware of your son's lackluster performance. I think it is interesting that you were not made aware of his problems prior to the report card, though. Ask her the reason for this.
Fourth grade is an awkward age.
The children are more independent than primary children, but not quite as independent as fifth and sixth graders. The teacher may have been working with your child to deal with this on his own to foster that sense of independence and responsibility. Once that has been established, share the information that you learned from your son with the teacher. She may have noticed some of these things already, but some of it may be new to her. Although it won't solve the problem with his behavior, it may help you and the teacher find a resolution together.