Drywall seams will crack or separate for different reasons. A damaged foundation can cause it, as well as improper drywall finishing during construction. It could be that the sheetrock hanger didn't use enough screws or nails. If you have a foundation problem you should fix that before spending time on your drywall. A foundation issue is serious; it will cause your windows and doors to bind up and can cause roof problems. When drywall is screwed or nailed to the studs, it makes for a seam every 4 feet...