My understanding of flexwatt is that you cannot mix different ones (3", 4" or 11") on one controlling device (i.e. tstat, rheostat). As well you cannot of course exceed the recommended wattage for your device. In other words if you have different flexwatts, each needs their own tstat or rheostat. That is my understanding of it.

I would really recommend you speak with Rich at Reptile Basics though. He is the expert on flexwatt that I trust. He does also have a very informative FAQ about flexwatt on his site. www.reptilebasics.com