There are major differences (ie. apples and oranges) between Cat4k and Cat5k in features and platform support. The cat5k is older generation product and I would expect to "go away" in the future. The Cat4k runs Cat OS or IOS depending on supervisor. The Cat4k is geared to near features such as Inline Power for IP telephony and a very long list of additonal features. The Cat5k will be supported for a long while, 3 years after end of sales.
