For some reason I always thought that Dolby NR was the same as Dolby B. I have never seen anything other than component tape decks with Dolby C.
As far as keeping a head unit for 4 years, yes I guess most will upgrade more than that, but it's nice if the old head unit is still working so you can sell it or give it away or install it in a 2nd car.
I don't doubt that head units of today won't last much longer than 4 years. Disposable society and all of that. At least with a factory head unit you know it's usually designed to last at least 3 years.
