Technically I don't think there's any reason you can't do that. I've heard of people doing it before for sure. There is a $10 per person charge for not showing up for a reservation, but that only applies if nobody in your party showed up. If you really had 4 people but one got sick and only 3 of you went, they wouldn't have a problem with that or charge you for the missing person. At least that's how it's operated to this point. And how's Disney to know that's not what really happened in this case? And likely if you had 3, they would put you at a table for 4 anyway because they don't usually have tables with only 3 chairs, so it's not like you'd be taking a table that you wouldn't have occupied anyway. Unless you have a moral opposition to it, I would venture that you could do it no problem.