the Lemans IV had the taller steering neck.
the Lemans IV had the rear frame rails bent up behind the shocks
they both had long swingarms, the Lemans III had the early smaller diameter, and 2.15" rear wheel
the IV had the fat tubing dimpled for the wider rear rim/tire.
none of this affects the exhaust.
They did have different foot peg hangers, but I know the Lemans III muffler had removable muffler hangers. I'm pretty sure the head pipes and cross over will work. You might have to fabricate a different muffler hanger at worst. .