I taught 4th grade for a number of years before staying home. The first thing I would suggest is if he doesn't already have an assignment book to get one and you will have to show him how to use it. Obviously, this is where he must write down assignments and projects. Next, I would definitely request a conference with his teachers. Just because this is a college prep elementary doesn't mean that kids don't struggle and that you have to do it all by yourself. Having a conference shows the teacher that you are most definitely wanting your child to do his best. The teacher may collect homework differently than he is used to (i.e. the student is responsible for putting it in a tray automatically). By meeting with the teacher(s) perhaps you all may be able to brainstorm some ways to help him get organized. BTW-this is not an uncommon 4th grade behavior. There is a HUGE difference between 3rd grade expectations and 4th grade ones. Much of the responsibility shifts from the teacher to the student as far as getting things turned in and remembering assignments. Our students had to have their assignment books initialed by parents every day (and I checked). It is really important that you check that book and whatever type of communication folder comes home. I am sure you already do this, but a lot of parents don't. Another suggestion would be to sit down with your son and discuss what is going on in school. Ask him: what he likes, how he feels it is going, what's going well/not so well, what does he need help with to be successful. If he doesn't already have a "special homework area" at home set one up. It could be part of the dining room table or a special desk for him. At this area should be school supplies, a dictionary, a little lamp and anything else that he would need. Let him help you organize it by talking with him by saying things like, "What do you think you will need to do your homework?" and things like that. Then together gather those supplies OR have him write a list for you to take to the store. This shows everyday organization to him. Have a special folder where the homework goes at night and pack the backpack before bed. Then, the next morning all is ready to go and the homework is in one place. Most kids don't know how to organize themselves (doesn't matter what their academic abilities are) because for so long there was no need to be organized.
As far as proper punishment--I would first try to get to the heart of the matter and talk with your son and meet with the teachers. Ask yourself, is he involved in too many extra curricular activities?? If so, cut back and choose one that is really important to him. Explain to him why this is being done and if school starts to improve and he is able to fulfill his school responsibilities than you can add another with the stipulation that if schoolwork falls off so does the activity added. And sometimes a failing grade is punishment enough for a student used to getting all A's. And remember 4th grade is harder so maybe getting some B's is not the end of the world. One other thing I just thought of--do the teachers have web sites?? If so, they may post homework and projects there. The district I worked in used this communication tool and it definitely helped the parents with this exact issue. If they do have a web site--then sit down with your son to go over the tasks. Have him transcribe a list of due dates to keep at his homework place and together check it every day. After you start to do this together awhile he is going to want to do it himself and it should become a habit for him. However, that does not mean that you still don't stay involved. If the teacher doesn't have a site, then maybe you can request that an email be sent to you at the beginning of the week with assignments and projects with due dates. If this is a college prep school the teachers should be organized at least a week ahead of time. Of course, things change during the week, but you will have a general idea of what is going on. Again, this type of request works best when conferencing with the teacher (in person) and brainstorming ideas for all of you to help him be successful. If this is the case, still sit down with him Monday after school and go through the email. If you are including him and seeking his help/input, it should not become nagging.
I am sorrry that my response was so long, but having seen this A LOT I wanted to give you some ideas that parents and I had come up with in the past.