Is one quad GPU rig better than two dual GPU rigs for Scrypt mining? I want to buy four r9 280x cards for Scrypt mining. I already have all other hardware components to create two dual rigs. Would one mining rig that uses all four GPUs instead be significantly better?

I would have to buy a motherboard or riser cards to create the quad rig. I am indifferent to power consumption.
Theoretically you should see a net saving in electricity consumption for there ought to be some economies of scale in having a single motherboard with the 4 cards vs. 2 separate motherbs. each with 2 cards each.

The CPUs controlling such cards are consuming power themselves but such consumption doesn't get much bigger if you have 1, 2 or 4 cards in the same motherboard. Plus the inefficiencies in AC/DC conversion within the power supplies would double, so yes - you should definitely see some savings with that arrangement.

But then, such a machine would consume tons of power, which not a lot of power supplies would be able to yield - 500 Watts, 800 W? Plus all the heat they'd generate, which of course needs to be dissipated somehow - which means not just some big fans, but probably some heatsinks, Peltiers or just submerging the whole arrangement in a bath of mineral oil -- fluorinert would be even better, if you can afford it.

Such big initial investment costs could offset part of the savings, so I believe it would be barely more profitable than the 2 machines way.