If it's better than 4.5, I doubt it will be by very much, but 4.4 is certainly not out of the question. After all, the new M3 has already been tested at that time from 0-60, so it could match that hopefully, at least in a test by one or so magazines
It's gonna have to be faster than that. If a large Chrysler 300C SRT8 sedan that weighs over 4000 lbs can run 0-60 mph in 4.8 seconds, shouldn't the IS-F be able to do it in 4.4-4.5 seconds? Besides, Lexus has already said "less than 4.9" and 4.9 is not less than 4.9.