You can twin turbo any thing, cost aside its just about getting the turbine right and the flow across the pump (compressor) right. All a turbo charger is, is a small turbine driven centrifugal pump. With a small 4 cyl engine you wont be able to use large turbos due to the lack of exhaust pressure, but two smaller ones and the right geometry on the turbine will work. You can even have each cylinder have its own turbo. A few years ago I read an article in a horse power mag (which one I don't remember) about a quad turbo V8 Camero. However with the technology in today's turbo the only benefit from twin turbocharging a inline 4 would be just to say you did it. The weight of all the plumbing for the two turbos I think would negatively effect the out come, with a single turbo would be lighter and less complex. I personally would go twin turbo just for fun, because I like doing stuff like that, that and make the car RWD.