I like the looks of the salvini, the site I looked them up on stated that minimum tank length was a whooping 6.5 feet though.
However they are bigger than Dempseys, why would they work better than a pair of JDs?
My LFS said a pair of Dempseys would do nicely in a tank that sized.
They're about the same size, maybe even a tad smaller than JDs in my experience. 6.5 feet is ridiculous. I wouldn't have any issues at all keeping a pair in a 4' tank. Don't keep anything else with them (a pair) though.... or it will definitely get pulverized.