DOOM 3 Alpha [2002] E3 Demo

 

This is the DOOM 3 Alpha Demo shown at E3 2002. As you can probably tell... the engine is pretty much the same but the gameplay has changed entirely.

DOOM 4 Demo

 

Found this video posted in Russian (I think). Some people say it's a DOOM 3 mod, some say Quake... possibly iD5 engine? No one really seems to know. It's pretty cool none-the-less.