I chose the 4 inch because I emailed the Reptile Basics guys and they pretty much confirmed my thought process and that is 4 inch flexwatt will work for quite a while. They also said that the worst case scenario was I would end up down the road using a 41qt tub and in that case I might decide to use two strips of 4 inch. If you look at a lot of the rack setups on YouTube, those guys are almost always just using 4 inch flexwatt and foil tape.