No need to re-invent the wheel. @RocketEngineer has already designed the perfect stand made from dimensional lumber. And if you skin the stand, no angled bracing is needed. Here's the design....and with a 120, top frame is 2x6 and all the rest are 2x4's.
And here's my frame for a five foot 90 gallon tank using this design:
In all honesty, 4X4s are not that good for this kind of thing. Because of the way they dry, they tend to split, check, twist, and other movements over time. A pair of 2X4s screwed together is a much better option. Not that the 2X4s don’t do the same, they just do it much less.