I know within my own batch (W12) the highest a startup pitched itself at demo day was about a 2.5- creating a company substantially bigger than Apple. I wouldn't be surprised if you could find a 4 or 5 somewhere in in all of YC with something along the lines of creating the next version of the internet (especially from the startups doing hardware). The tradeoff there is the variance is even higher; doing a software startup is safe, relatively speaking!