Not even sure there's much difference between 3000, 4000 and 16000 in practical terms. They all just mean the computer crushes you and laughs in your face.
I was running through one of my games on Fritz a couple of weeks ago, and out of the blue it announced forced mate in 25 for me, starting with a move that I had dismissed as being bad. I guess the only difference might be that the 4000-rated engine would announce mate in 50 or something like that
I read this somewhere, and apparently the "perfect" chess computer would have an elo rating of approximately 16,000. Since the highest current rating any man/machine has now (the year 2010) is about 3400 from Rybka 4.0, we still have a long way to go.
That doesn't make much sense, elo ratings are not absolute measurements of skill, they just show how well you perform against others in the same rating poll. So a 16000 rating would be pretty impossible to get by playing against 3000ish computers, even with a 100% win ratio-and even if the computer played enough games to rise to 16000, all we'd know is that he has a perfect score against 3000ish engines, but that could be a skill level of "just" 4000.