Supposedly the M4 will be the coupe while the M3 moniker will migrate to the sedans.
From the linked page;
"The new M3 & M4 will use a 400Bhp V8 due on sale in 2006 starting at £45,000 at todays prices, also planned is an Estate (Touring) version to take on it's rivals like the Audi S4, Mercedes C32 AMG & forthcoming C55 seen as a large market share for Tourings. The V8 version is likely to be a 4.0 Litre with drive to the rear wheels & update version of the SMG Sequential Manual Gearbox through the traditional 6-Speed manual. Although heavier than the tradition 6-cylinder engine it will be off-set by the lighter hybrid construction on all new 3 & 4-Series.
The new M3/M4 will be based upon the forthcoming 3-Series due out in 2005 codenamed E90."