As of THIS year's sales or total sales ongoing? The answer for both is 3 years quite obviously, but it will change what month it accomplishes the goal if we're talking about future xb1 sales as well.
Saying 3 years also doesn't imply that the Switch will outsell the PS4, that is a ridiculous statement. The PS4 sold around 50 million by it's third year. I just think Switch will get to 35-40 by then.
Also don't see how 4 years is reasonable to be honest. Edit: Ok maybe it's more reasonable than I thought...