Wow, that's almost exactly what I did for my 55 gallon, but using 2x4's instead of 4x4's. I also put some trim around the doors, and I also put a shelf on one half to make some more room for all the small stuff.
Pine has a compressive strength of 750 psi, so wood is more than strong enough to hold a tank. You've just got to be careful and not let it sag in the middle, so you do need an extra brace there. Also need to keep in mind how the weight is going to be applied. That's why they make the rectangles for the top and bottom, then place the legs between them. Distributes the load across the whole bottom instead of just at the four corners. (Damn engineers
) But mine is stout as a tree. Soaked the 2x4's in water seal, then added the "skin" on the outside. Stained and varnished to finish it.
Need to get some pics posted.
But no, you really don't save any money as compared to buying a prefabbed one, but there is that peace of mind!
I wouldn't buy any that I've seen being sold so far. Too rickety in my opinion. It also took me about 2 months working on it after work and on weekends.