DeWitt P4 4'x250' Pro 5 Barrier 5oz Fabric
Pro 5 Barrier is the best bet because it is 5oz (rather than 3oz) and you get 250 feet in a roll.
Here's the 6 foot wide 5oz version...
$176.21 + tax
The price is obviously more than the 4' version, but as there are less join overlaps it is a little more efficient.
You can sometimes find it new on eBay at http://www.ebay.com/itm/Dewitt-P6-6-x-250-Pro-5-Weed-Barrier-Landscape-Fabric-5-ounce-weight-/272274097461 6'x250' $150 + $0 + $0 This is by far the best bet.
For lining the bottom of the footings it is the most efficient to use the 5' wide variant. On eBay it is $125 instead of $150.