Cedar 2x4 deflection across an 8’ span
If you were planning on a single front rail you can expect substantial sag over time. The cedar would probably deflect 1/4" to 1/2" initially, but will continue to deform for the first several months. 

I'd install at least one additional rail behind the front one, probably using pressure treated pine. A 4x4 wouldn't be excessive. Be sure to crown all rails upward.

Alternatively you could screw a full length 1/4" by 3" aluminum plate to the back of the cedar rail using pairs of 1/4" by 1-1/4" lag screws at 12" intervals. 

If you don't like that plan a single leg fastened well to the front rail should do, or extend the two shelf support verticals to the ground, notching around the front rail.