Jasey said:
http://fastestlaps.com/comparisons/bmw_m4-vs-bmw_z4_sdrive_35is.html On the track tests it's 4 seconds quicker (for a 1m 40 second lap).
Both cars are in that bracket of "almost supercar" fast. The M4 especially. However the Z4 35IS with the roof down and the foot floored adds another level of excitement that the M4 can't. More importantly it's a level that can be enjoyed easily on public road, even in the South East. The M4 by comparison feels like it's doing half the speed it actually is and can only be fully utilised on a track. The difference in lap times shows that the Z4 holds it's own very well for a car that's carrying so much luxury and is not an M.
Lets face it, the M4 is an out and out ballistic missile. It's also incredibly luxurious and can be specified with enough toys to make the Starship enterprise blush. It is the full package and potentially the best all-round car money can buy. I think it's that good. The Z4 brings something else to the party, it's smaller, the roof goes in the boot, feels more personal, you are cocooned in the cockpit and the noise of the engine / exhaust is all around you. It's more fun, slightly less serious.
But we are (IMO) comparing two amazing cars, either of which you can't go wrong with. Unless of course you have a family of 4 and need a large boot