Deep Blue makes some nice smaller tanks. They are fairly inexpensive, depending on where you go. I think I have the B40 or B4 or something like that. it's a rimless tank.
I just ran the calculations for building a four gallon tank, and this looks like it is 4 gallons. It doesn't look big enough for four gallons, but apparently it is. It should be less than $30. They also make a small filter that works decent of you throttle it way down. It's a bit loud for my preferences, but it's in my bedroom and I thing the dresser amplifies the sound.
Why not go with acrylic if you are building it yourself? 9.75" per side.