4 pounds weight loss! One good trip to the John might let you download that much! Heh!
But that's a good question! I see a lot of nice carbon fiber bikes on rides these days and their owners sure swear by them and think they do better with them. Like Stepfam says, not necessarily more speed, but fresher at the end of the day and better climbing ability. And I've never met a carbon fiber driver that wanted to go back to their previous steel frames with their weights.
I'm looking at dropping about 15 - 17 pounds from my steel LWB bent as soon as I'm convinced it's worth 3 or 4 thousand bucks! Again, not so much for speed, (I do fine on the flats and overall) but for climbing and end of century freshness. But bents are different and weight is less important, and 15 pounds in them might equate to about 4 on an upright in terms of 'lightness.'
So this discussion, while centered on uprights, is very interesting and informative for a crippled-up bent up old benter like me!.
Also losing 4 pounds might make a bike feel more nimble and make the ride itself more appealing and more fun.