Tempered is 5x stronger, but if it goes, it's a big bang, no warning, no cracks. I think most aquariums that are larger stick to tempered glass, for a piece 6' x 2' of 5/8th" i got a quote of 180$ (all non-standard sizes of tempered glass are special order since it can't be cut) so you may want to see if you can get a price on that before deciding, it would appear the front window will decide what the tank is made out of.
1/4" acrylic is way too thin, perhaps ordering online you can find a cheaper place with thicker acrylic?
if they charge down to the square inch, you'ld be looking at 140$ for the front glass in 1/2 " (tempered or not, the price was yours)
Reducing the height of the tank to 2' would allow for 3/8's glass for the face (and that would be very strong)
http://www.sdplastics.com/sdplas2.html
Prices on all kinds of acrylic. (and all thicknesses)
Ah well, some things for you to think about, let us know which way you decide to do it, and take pictures.
I was gonna build a 150Gal, but no matter which way I sliced it, it was cheaper to buy it pre-made. :S
edit for your edit
I would use a bit thicker face material, 4' is quite a span to allow to bow, perhaps 3/8" .
Glad to see you found an answer
Oh, if you use glass, you can paint the galss before you put the wood behind it, this way no paint in the tank and a nice black background (or would this make a mirror?)