Porsche is working on new twin turbo 4-cylinder engines up to 360HP. Here's one article:
Honoring Heritage: New Porsche Flat-Four Displaces 2.5 Liters, Makes 360 HP - Rumor Central
The article says in part "And can we imagine a 911 with a four-cylinder mounted on and behind the rear axle? Answer: As much as we can see sport sedans, SUVs, and mid-engined roadsters wearing the Porsche crest. Don’t expect a complete drubbing of the famous high-output 911 flat-sixes though."
The U.S. is mandating 35.5 MPG fuel economy for new cars by 2016. BMW already has a TT4 for sale, but it's not a boxer.
So how about a 992 with the 4-cylinder boxer?