No, 4" tape will not even be close to enough to heat up a 41qt, you need the 11" wide stuff for that. Also with different sizes of flexwatt you'll need separate thermometers as different sizes heat up to different temps on the same power levels. I suppose you could always "throttle" down the hotter areas with dimmers but just be aware.

Do some reading and research for yourself seriously, there are over a dozen DIY stickies and they all have great info.