I've found this,
"However, BMW sources at the media presentation of the Concept M4 coupe confirmed to Autoweek the new M-car will adopt inline six-cylinder power, bringing an end to the reign of the high-strung V8 used by the last M3 after a run of just six years. Known internally under the codename S55, the newly developed twin-turbocharged 3.0-liter straight six is claimed to kick out around 450 hp, versus the 420 hp of the current naturally aspirated 4-liter V8.
Drive will be channelled to rear wheels through the same seven-speed dual clutch gearbox used by the more powerful 560bhp twin-turbocharged 4.4-litre V8 powered M5 sedan and M6 coupe, cabriolet and GranCoupe.
Nothing is official just yet, but BMW insiders hint the M4 coupe will accelerate from 0-to-62 mph in under 4.5 seconds and reach a top speed limited to 155 mph. "
http://www.autoweek.com/article/20130815/carnews/130819908#ixzz2c6scSIxC
A bunch of sources all say different things, but they all say in-line 6 and around 415-450 hp
They may all be wrong and doing it for attention though..