Wasn't the GT4 engine used in the Lupo mini-game they made for the Lupo racing team a long time ago? And if that was the GT4 engine, why would all the screenshots since then not be the GT4 engine? Usually, the engine is the first thing you build in a large game, so that you have a foundation to work with. Why would PD build cars and tracks before finishing the engine?
Edit: They are probably still PERFECTING the GT4 engine (like adding the upgrades to the AI, or implementing anti-aliasing), but I don't think the graphics in those screenshots are being produced with the GT3 or GTC engine. I could be wrong.