I bought regular 4" and the Dust Right expandable 4" through Rockler. I have purchased most of my fittings through there, but they can nickel and dime you to death. I'm sure fittings would be cheaper online, but I usually don't have the patience.
I use the heavy duty FleXaust that Woodcraft sells. It's clear so if there's a blockage, I can easily find the problem area and it's made in the USA. [thumbs up] I've used the 4" version for the last 12-15 years with no issues. I also use the internal heavy gauge spring helix wall support as a ground path back to the dust collector to eliminate static charge issues with the hose.
Oneida's hoses are the strongest and best out there; more expensive also. They are reinforced with a spiral steel ring and you really can't damage them. I've stepped on mine a couple of times; no damage. They would have 4" and I would think 6" (but I don't have any 6").