any who has 4 cell. Will a single charge last more than 5 hours? approx how long?

It probably depends on how dim you can tolerate the screen. I get about six hours on the six-cell with the screen set to half and WiFi on just doing normal stuff.

do you know how much you gain in upgrading to 6cell and what's the incremental weight?

I don't have the four-cell battery, but with similar usage, I'd expect four hours from it. That means you'd get a couple extra hours from the six-cell. The six-cell adds about 1/3 of a pound in weight.

I never have good luck in 4 cells.. I turn my brightness all the way up, so 6 gets me about 2:30 and 9 gets me about 5

The 4 cell is good for 4 hours tops. The 6 cell is over twice as much capacity. 63Wh vs 28Wh.

it's not much bigger than the 4-cell, so, I'd get the 6-cell. It only raises the rear of the laptop by a little bit. As long as it doesn't stick out the rear then it looks just fine.

some actually prefer the 9-cell since it'll provide so much battery time. BUT, with the 4-5 hours of the 6-cell with screen brightness at a manageable level, I think it's a good trade off.