I've never spread the rear triangle of a frame from 126 to 130. Steel, aluminum, carbon fiber, it never mattered.
As has been said, you're talking about the width of a spoke (2 mm) on each side. There has never been a frame that wasn't designed around way more than 2 mm of cyclic deflection at the rear dropouts... because you're gonna get that with every pedal stroke unless you make your frame out of pressure-cast concrete.
How to put a 130mm hub into a 126mm frame? Stuff the rear wheel into the bike, set your high and low limit screws, and ride the bike for the rest of your life.
--Shannon