Paul,
 Not sure if they are the lightest but I just weighed a Turnigy Nanotech 3.0 ah 4s at 289 g(10.19 oz.)available from Hobby King. A Thunder Power 3300 mah 4s at 328 g(11.57 oz.). And a Turnigy 3.0 ah 4s at 360 g(12.69 oz.). I like the Nanotech at it's low price and respectable weight.
Bob

Keep in mind that it is all about energy density. You can safely discharge a lipo to about 3.75 volts per cell without damaging it. The turnigy lipos sag under load tremendously, yet the Thunder Powers hold up to an increased draw better than most. This affects useable capacity. I have been doing electric RC since day one, and yet flying electric control line has taught me so much. My favorite lipos are the 25C Hyperions- flying back to back flights with the turnigy 2650's and Hyperion 2500's yields me more usable flight time with the Hyperions.And here is the fascinating part, that I never noticed flying RC- you can FEEL the additional power they have.

Yes, they work very well.
And they even put your name on the label....

(I use the Black Label 3900 and the Platinum Label 3700 as they weigh the same. I've noticed no difference between the 30C and the 40C in performance.)