I have my 41s on 4" Heat Tape, however my room stays at 80 degrees. As other people have stated, it all just depends on what your room set up is like. If the room your reptiles are in is 75-80 you probably only need a 4" piece. However if your room is on the cooler side you will want the larger piece since you have more of the tub to make warm. Hopefully that makes sense. :D