I would tend toward overbuild.
If you are simply building a deck, spanning 2x4's 4' apart would give you sag in no time. And a 70 gal
is a lot of weight.
Spanning 4' would require minimum of 2 2x6's on edge if I build to code. I would go at least 2x6 if not 2x8's in a multi-level fish tank. If that is too "thick" you can use angled iron (or I-beams) to stiffen up the 2x4's. My lfs
has multi-level tanks as well & the stands are metal.
Also, I would not just use a single 2x4 notched like you planned. 2x's have lots of compressive strength, but not enough lateral stiffness. I would use 2 2x4's glued in an L for the verticals. <Or use 4x4's.>
BTW, my stand was built using 4 L-shaped verticals as above. The horizontals consisted of a 2x6 glued & screwed to a 2x4. <Giving me an L-shaped "shelf" so I can rest my tank inside the beam.> All joints are half-lap or bridle. This is prob overkill, but is stable as heck. I have an 80 gal
on top & my 30 gal
sump in bottom, plus a 10 QT in the bottom if needed ... so this would count as a multi-level tank stand structurally.