The first two pours set sweetness vs. acidity.
In the 4:6 method, the first 40% of the water controls balance: a smaller first pour tastes brighter, a larger one sweeter.
What it is
In Tetsu Kasuya's 4:6 method, the first 40% of the water is split into two pours that shape sweetness and acidity.
Why it matters
A smaller first pour and a larger second one gives a sweeter cup; a larger first pour and a smaller second one, a brighter, more acidic cup.
In practice
Start with equal pours. If the cup tastes too sharp, make the first pour smaller next time; if it tastes flat, make it larger.