How does Black 4.0 feel to use? How thick is it? Does it need to be thinned to work properly? Is the finish smooth or chalky? I'm intrigued.

The mask looks great, love the contrast.

I use the paints and pigments created by culture hustle. Working with black 4.0 is really easy. It doesn't need to be thinned. Nice somewhat runny texture. The finish isn't chalky. It's a smooth mat but still delicate.
I would recommend using black 4.0 over black 3.0 it has a chalky finish. The best way to use it is to try to make the layers thin as possible. To be able to get the darkness acquired. The mask only has two layers of paint. To be able to achieve the darkness you have to use a hair dryer. It takes a a long time to dry alone in the air. Using hair dryer is highly recommended doing achieve the darkness.
Here's a link to the culture, hustle and all they're amazing products created by their CEO, Stuart simple. He actually likes some of my art on Twitter when I use 3.0 and 4.0 black.
https://www.culturehustleusa.com/

Thank you for the detailed explanation, it was very helpful!