You’re right about what the parameters mean, but unfortunately, the thumb keys aren’t in row 4, because there is no row 4. The left side thumb keys are column 7, and the right side ones are column 8. Row 0 on each side is the innermost key, and row 3 is the outermost key (closest to you, furthest from the finger keys).
So from left to right, they’re R0C7, R1C7, R2C7, R3C7 on the left side, and R3C8, R2C8, R1C8, R0C8 on the right side.