Why isn't runway 4L/22R at KMDW available as an emergency runway?
Prologue: the quote in the question is from Wikipedia, and the parts there that mention the designation to commercial aircraft emergencies have later been marked as "citation needed". Therefore, for as long as an authoritative source can not be verified, it might well be that in reality there is no such designation.

Actually, should the nature of the emergency require, any runway (or even taxiway) on any airport is available for any aircraft if the crew so wishes. Some more or less famous examples of such situations (Air Safety Network):

Air Canada flight 143 (The Gimli glider, fuel starvation)

United Airlines flight 232 (Sioux City, total loss of hydraulics)

Air Astana flight 1388 (Portugal, reversed aileron cables)

If you are out of fuel or experiencing severe control issues or something of such magnitude, regardless of what you are flying there is no way ATC is going to tell you "sorry you can't land on the 4L, turn to heading..."

This might seem like splitting hairs, but that's how it is. To state that some runway is (or is not) available for emergencies of a certain aircraft type, is somewhat of a moot point.

After you've spoken the three magic words, the ATC has to, and more importantly, they want to do everything they possibly can and more to help you out. You are finally free as a bird, if only you did't have an emergency to deal with...

Speaking of splitting hairs, Gimli glider did not land in an airport at all, but it's the most famous case of the sort...