If it was me I'd go with one 8' main tank and one 4' (refugium/whatever won't be happy in the main 8' tank) tank with a bulkhead between the two(from the back of one to the back of the other so you can butt the tanks right up together). You'll be able to create 2 very different ecosystems that way. drain to the sump from the end of the 8' tank and flow the return back into the 4' fuge so you don't have to worry about micro bubbles at all in the 8' and you'll keep a steady flow of copepods coming that way too...
Just a thought
Evan.