Verified answer

September 15th, 2008. It says on the side of the page

http://tviv.org/Weeds/Season_Four

Weeds season 4 has 13 episodes and the 12th episode is airing tonight September 8 th.........so the finale will be September 15 th.

it will air sept 15, 2008 and i am guessing that it is going to be great! i can't wait