The 8ft tubes are nice because they have a large bright area , away from both end caps , you get less lumen's as you near the ends of the tube.
The 8ft. are actually rated exactly double the lumen of the 4ft. tubes.
Some Philips 8ft here being rated at 5600 lumen , a 4ft produces 2700 lumens.
http://www.usa.lighting.philips.com/...ds/p-3554d.pdf