Many of the things we do will have to take into consideration what we are set up to do correctly. The design with using more wood at the corners can be the correct method if we are short on tools but if we can cut a 45 degree miter, which is an easy thing with a cutoff, miter, radial arm, or table saw, but might be questionable if we are trying to cut a miter with a handheld saw like a circular/"skill" saw as the miter might not be true. But the same is true if we don't cut the ends totally square when cutting 90's! So using the right tools can be very important to how strong the result.
If we can cut a decent miter, laying the top boards flat, cutting 45 at the corners and screwing and gluing the mitered ends can let us then lay the top on two 2X4 glued and screwed together to form an angle for the uprights. One of the things I like about this is the speed of building two sections (top/bottom) just the same and then adding them to the uprights. The only remaining boards to add are whatever we feel is needed to frame for any doors and as a way to assure the top and bottom wood can't twist or bow in any way. I add 2X at each edge of the doors as it makes the door attachment solid.
At the joint between top/bottom and uprights, some thought about the exact placement will get a better joint. Look at the way one portion of the upright can run under both boards of the top and you can see that one single boards should support both boards of the top and that keeps either part of the mitered top from drifting up or down to weaken that joint. Basic construction thinking tells us that it is always better to stagger the gaps at joints so that each section is supported by the next. That is why brick is often staggered to get better strength.
If one has the standard tank that uses the frame of the tank to set on and the glass is actually not down on the top, there is little value in adding a solid surface as it supports no weight at all. Most tanks only set suspended on the rim! Using good grade plywood to keep the top from warping is just a feel-good project.
When making joints that I do want to stay put, I do highly favor a good wood glue that soaks into the fibers and makes the joint stronger than the wood in many places. I also use long screws, both to speed assembly and to add some strength.
Cut four #1, four #2, Glue and screw them together with long screws at an angle to catch both boards. This makes top and bottom. If the corners are square, you've done it right!
Cut 4 #3 and 4 #4 or 8 of the same ? Glue and screw them together to form four corner uprights. At the same time you are cutting the uprights and have the jig set, cut any added supports for uprights at the door or more to firm up the top if it is a long tank. Cutting them all the same is a key point!
Set the top on the corners support so that one board of the corner overlaps the miter, glue and screw it on at all four corners and add the extra uprights.
Skin it and add trim, etc. to suit whatever taste you want.
This design will hold any size tank at any height you want. It is so basic and simple that the only change needed is cutting the boards to different lengths and possibly adding more uprights for tanks which are much longer. It has the additional advantage of having lots of wood at the floor, so that it is less likely to mash into any of the soft floorings we might put it on. It will still mash carpeting under a big tank but less that if we put the thinner side down. KISS it and be done!