It sounds like you are commuting to school so I presume you have a time schedule. I am a working schmoe but flexible in hours so I typically arrive at work by bike by 7am and leave at 4pm, thereby avoiding a lot of rush hour. If you can work that in your schedule, you could alleviate the high traffic areas being high in traffic (which I presume a 4 lane road would be (2 lanes each way)).

Also, you might start checking out google maps with the satellite view. I obviously don't know the area you commute in but you'd be surprised at the amount of informal bike paths that you might be able to use to avoid some of the road walks. In both my jobs, I have been able to find shortcuts through the woods or places where cars can't go that would go around the high traffic areas. Sometimes you can spot them using the satellite view...

barring that, the other folks in this thread have good ideas... More time will give you more confidence/comfort..

Jay