Even calling him Detroit's number four is bad. You've got Max Scherzer, David Price, and Justin Verlander, all Cy Young winners, and no matter how much Verlander is struggling he's still Justin Verlander. Then you've got Anibal Sanchez, who in my opinion is the best out of these four and maybe second in the AL behind Felix right now. Porcello is a five, and he'll be the odd man out if they go to a four-man rotation.
It's not even necessary to have a rotation during the postseason. You can have five players designated as starters, or you can have four with the fifth guy out of the bullpen. At some point, someone's going to try to go with three and have two starters turned into long relievers. It sounds crazy, but if you have the right three (maybe the Dodgers or Tigers), it's doable. Given that there's a bunch of break days during the postseason, having a rotation is at times counterproductive - if your best pitcher has it in him to throw 100 pitches at full strength on three days rest or on six days rest, whatever it takes, throw him. Adam Wainwright, Clayton Kershaw, Felix Hernandez - these guys could throw every day if you asked them to, and I'd bet a lot of money they wouldn't say no in the postseason. (I'm not advocating for that, by the way; that's a bit ridiculous.) I'm not sure why the teams with names like theirs even bother telling their other pitchers when they're going to throw next - give him a day to throw, an off day to golf, a day to run, and a day to throw again. Game 1, 3, 5, and 7.