Is Extreme Wolfenstein 3D v4! free?

Yes. Extreme Wolfenstein 3D v4! is released under Free fan mod (see the ModDB page for terms) and costs nothing. Neither it nor Wolfenstein 3D via ECWolf has any paid content.

What do I need to run Extreme Wolfenstein 3D v4!?

Wolfenstein 3D via ECWolf, which is itself free. Extreme Wolfenstein 3D v4! is an add-on and will not run standalone.

How do I install Extreme Wolfenstein 3D v4!?

The PlayBound Launcher installs it in one click from Wolfenstein 3D via ECWolf's mods list. You can also install it by hand — full steps are on this page.