Delta KJ--if you are going to have a 3 x 7 bench i would suggest a leg at midspan---the bending moment on your 4x4 supports is pl/4--so at a 7 ft span for say a 1000# load the bending moment would be 7' x 12" x 1000/4=21000" lbs. The section modulus of a 4 x 4--assuming dressed lumber would be 3.5 x 3.5 x 3.5/6=7.14 in cube

the bending stress would then be 21000/7.14=2941 psi- --you really don't want to go higher than about 1500 psi for bending stress on timber.

24x72x1-1/2''thick plate 3'' tube legs with shelves lenghtwise to slide my many press brake dies into and a big 6'' vice on one end hyd. punch on the other. I est it weighs around 3500-4000 lbs Works great for banging on what ever and back up to hold a jig while bucking 5/8'' rivits. One of my buddies has a bench ~4''thk 3'x6' we used it as a deadman while setting his 50w'x20'h steel roof trusses on his shop.

Depen. The one I mentioned that was 10 ft. long at the shop did have center 4"X 4" posts same as the ends with cross supports & yes they are really needed for HD work like tractor repairs etc. If it,s just for wood working or the like then such a HD bench is not necessary & just a 3/4" plywood top is all that,s needed for wood working & maybe just 2"X 4" legs is heavy enough for that bench. Out in the main shop also where I worked had a real HD one that was about 14ft.long with a steel top over the wood top & had a big vice bolted to it.You needed the fork lift to even move it at all.

SJ--i agree with you that for wood working a lighter bench would work---even 2x4 legs will hold about 1200 lbs before they start a buckling failure. the cross supports to the legs resist buckling and a safe value to use for compression perpindicular to grain is about 300 psi so 300 x 1.5 x 3.5 = 1575#
this is for say SPF lumber---certain hardwoods go to much higher allowables.
The south american hardwoods are unbelievable in their strength.