Making the comparison between a piece of furniture that can hold a similar weight in people to something holding up a tank is a false analogy. Put 4-5 180 lbs people on a couch--sure it will hold. For a while. And the people only sit there for a while. So, the weight is a temporary load and usually fairly stable. This is not the same scenario as a fish tank--that holds the same weight, 24/7, for years at a time. The weight fluctuates (water changes, re-arranging, adding decorations, etc), there's a good chance of water exposure, etc. Apples and oranges!
I agree, using 4X4 is over-engineering. Wood is sturdy stuff, and properly built, can hold tons, easily. However--using stronger material than absolutely needed is better than using inadequate woods. I'd rather have 4x4's, know it's more than I need, than use 2x2's and wonder.
For what it's worth, I have only 2 tanks that are NOT on stands I've built. My 40 is on a stand made of 2x4, the 55's are also on 2x4s. The SW system is on a cabinent that could withstand several close range hits from a grenade and weighs about 600 pounds. Over engineered? You bet! There's well over $4000 worth of rock, corals, fish, and lighting on that puppy. I want it to be solid.