Yayy, finally a chance to break out the
Quake family tree picture!
As you can see from this image, both the IW engine and the Source engine are ultimately descended from the quake engine. They may as well be called Idtech 30A and Idtech 30B, but they aren't because major changes to an engine sometimes necessitate a change in name as well. What's more, today's Source engine is no more the source engine of 2004 than the 2004 version was GoldSRC, or GoldSRC was Quake 1. Similarly, IW 4.0 is not the same engine that they used on CoD 2. Companies have just started marketing more on the name of the engine than they have the version number; think about it, the Source engine is a really famous engine. If you were valve, would you throw out that name recognition just because you put out a new version of the engine?