At 4 feet tall? I would go no less then 1" thick on the glass. That's tempered, 1.25" on regular glass. Maybe a bit thinner if you are euro bracing, better safe then sorry, very sorry.
Calculator is not for the tempered glass right? I think you have to divide what comes from the calculator by at least 3 to derive the thickness for tempered glasses.
This is what I got from the glass thickness calculator of http://www.garf.org/tank/BuildTank.asp
Material = Glass
Tank Height = 36"
Tank Width = 48"
Tank Length = 84 "
Glass Thickness = 1/2"
Approximate Gallons = 628
So if a 3 feets high tank is okey with 0.5 thick normal glasses, 4 feets high tank should be okey a 0.5+ thick tempered glass, as I think.
It doesn't allow to calculate thickness for glass tanks with heights over 36", saying glass tanks must not exceed the height of 36". That couldn't be true though mathematically.
If u feel good about it do it but i my self woulnt put that in my house I would go for overkill with that much water and chance of damage if it where to fail that's just me tho