What do I mean? Didn't you read the link I posted?
2-stroke exhaust is designed to push a portion of the exhaust back to keep the combustion chamber filled. Strategic backpressure.
You can cut the muffler off of that exhaust and slip some flexpipe over it to make an exhaust pipe for your 4-stroke tho. A small seatclamp to keep it in place on the header. Chinagirl exhausts make awesome headers and they do bolt up to the 49cc 4-strokes. I did that on mine.