I just bought a TD-125 and I would be lying if I said this thread didn't scare me.
I was going to go for a 5" to 4" reduction, too, but now I'm glad I ordered 5" ducting online. I will be running it on low power as well, hopefully it doesn't end up dead like the couple others up there!
The TD-100 has a thermal "fuse"... once the fuse has blown, the fan is pretty much toast, unless you want to take the time to disassemble it and bypass or replace the fuse. I took the time... and my frustration... out on the fan and got it working again, but it wasn't an easy task getting it apart!
One thing don't run inline fans with the shaft vertical you want the shaft horizontal as the bearings aren't designed to be side loaded and the fan will fail prematurely. A lot of folks just slap a fan on top of filter with the impeller vertical and then bitch when after six months the fan dies, worst thing you can do short of throwing sand into the thing.