Looking at +$25k over MSRP at the moment to get your hands on one.
If you already have a Z4MC, you're have the next best thing to a GT4. At a fraction of a price as well.
/DRIVE had a show that discussed the GT4; and in several occasions during the show they noted the Z4M being a very good drive.
At the $100k price point (GT4 MSRP + premium) and if I actually had money to burn, I'd wait for 997.X GT3's come down a little more in price and try to snap up one of those.