Just came back from "The Source A/V" in Torrance, CA. Had a chance to try out the LCD-3 and LCD-4 with same source/amp. LCD-4 is definitely a much more improved LCD-3; thick bass, more forward treble, less dark of a mid. There is just a hint of that Audeze house sound leans toward warmth (some would think it's dark). Definitely better separations than the LCD-3. A pluck of a bass string doesn't get muddle into the singing voice. A Stax SR-009 was also there, so I think the LCD-4 is getting close to the Stax neutral mid; but the bass is not as fast. Overall, I think it's a 15-20% improvement over the LCD-3; it's definitely noticeable. I have the LCD-3F, given the LCD-4's price, I'm happy with my LCD-3F for a while.
On the weight issue, I never had problem with Audeze 'phones' weight. LCD-4's weight didn't bother me a bit.