JR I am assuming that the carpet was glued down, if so your 4" strip is caused most likely by a seam tractor. A seam tractor is a tool that blends the yarns together during seaming, it's common size is 4" wide, it is rolled along the seam. This could be a possibility as well.
Please give me some more detail. 1) what is the fiber. (NYLON,ECT)? 2) Is it a level loop, cut pile ? 3) Is it glued or over pad?