I think the Samsonite Cosmolite 20" is the lightest 4 wheeled hardside carry-on.
There are two versions of this bags, the original one and the updated one, and the current version is slightly lighter than older version.
But in some countries, including US, they don't offer new ones.

Here are some specs of the updated cosmolite carry-on.
1.80kg(3.97lbs) / 36L / 55x40x20cm(21.7x15.7x7.9")

http://www.samsonite.co.uk/cosmolite...?or=3807161717

Light as hell. In fact, even the 62 linear inches cosmolite often feels lighter than other brand's small carry-ons.