I would suggest you skip the 4x4's... you'll have warping/twisting issues as the wood dries out.
I used these plans for my 75g stand and my 30/20 stand:
DIY Stands Template and Calculator - Reef Central Online Community
I made mine 40" tall so I could see it at eye level easier. Sounds like the same thing you want to do. No center support needed if you follow the plans. I think they say in the thread anything over 72" span needs a center brace.
I like these plans because the wood takes the load, not the screws. It can be done cheaper and using less wood but the way the plan is laid out it really makes cutting everything easy. It is all color coded and you can do all your cuts at once. I did add some braces at the bottom and covered the bottom with plywood also so I could use it to store my canister. I used 3/4" plywood for the top.
Here's my 75g one:
meegosh's 75G Geophagus Tank