According to this online aquarium glass thickness calculator, 10mm glass should work for a tank that's 10'x4'x2'.
http://www.theaquatools.com/building-your-aquarium
A glass tank that size is going to weigh over 500lbs.
I would try to find a local glass shop/aquarium builder to get an estimate.
Around here a brand new glass or acrylic tank that size will break your budget several times over.