Diopsids have no special rules for multi-handing a single weapon, so they'd go to the default rules for it, which I believe are that you add .5 Str for each hand after the second, though don't quote me on that. Given that, I think that means a Diopsid would be getting 2.5x Str on a weapon if they quad-handed it.
Yes, those rules are from Savage Species, page 42.