Since the diffuser that's made for the Sedu 4000 has been out of stock, I got the Hot Sock since it's practically the only one that will fit on it. It was less than $5 on Folica.com and I'm really impressed. It's just a light, foam mitt that goes over your dryer -- you are supposed to leave 2-4 inches hanging -- and you use it like any other diffuser.
Because the nose of the Sedu 4000 is so short, I have to leave the concentrator nozzle on and put this on top of that. This combo dried my hair super fast and left me with my natural wavy look.
It's a great tool for days when I don't want to take the time to do a blowout, and this won't take up any space at all in my suitcase.