3x3 in your link is exactly same size as 4x4 in initial post. It's just difference whether you include required amount of space in the dimensions.

Edit: I was wrong. Some of Simplifier characters like j actually use full 4 pixel height and poke out of 3x3 block.

I get it, no problem. Some characters are extremely hard to make them fit in 3-4 pixels and I cheated on some too!

Looks like the text on some of the modern Atari 2600 games.

Simplifier's 4x4 includes both upper and lower case.

Yes, I was simply experimenting on how low you can go. I love small fonts! And Simplifier's one is clearly impressive.

Hats off to your experiment, it's still impressive :).