“Mad Men” fans can start speculating about the titles of the last four episodes for Season Six. This Sunday comes “A Tale of Two Cities,” followed in order by “Favors,” “The Quality of Mercy,” and “In Care Of.” There’s no other information about them so far. It’s possible “A Tale of Two Cities” will be about New York and Detroit, where the Sterling -Cooper-Draper et al. ad team is trying to deal with their Chevrolet account. (They’re working on the Chevy Vega.) The show is now up to mid June 1968, so it’s a little early for mention of the Chicago Democratic Convention in late August 1968. But nothing is ever explicit on “Mad Men.” Last Sunday’s episode was so well done– indeed, the last four or five were so amazing– that we’ll have to wait and see what they have in store for us.