Here is how brush width works with Antialising Disabled in 4.0:
Brush width 1: Brush is 1 pixel
Brush width 2: Brush is a 2 x 2 pixel square.
Brush width 3: Same as brush width 2
Brush width 4: Brush is a 4 x 4 pixel square minus the four corner pixels.
Brush width 5: Brush is a 4 x 4 pixel square
Brush width 6: Brush is a 6 x 6 pixel square, minus the four corner pixels
Brush width 7: Same as brush width 6
Brush width 8: Brush is a “circle” that is 8 pixels wide
Brush width 9: Brush is a 8 x 8 pixel square, minus the four corner pixels
Brush width 10: Brush is a “circle” that is 10 pixels wide
Brush width 11: Brush is different “circle” that is 10 pixels wide
So it is impossible to draw with a brush that is 3, 5, 7, 9 or 11 pixels wide. And there are two identical brushes. Not sure if these issues are bugs, but it is unfortunate.