While keeping our engines above 4k does keep carbon from building up, it does wear the engine out sooner too. Brucelee stated this elsewhere on the forum a while back when asked this same question. Good logic.
I don't keep my motor above 4k at all times, that's for sure. But I do run it up to redline most every time I drive it though to blow out any cobwebs that might have accumulated in the car between drives
