Use Bohr's radius formula
For hydrogen atom, the radius of the $n^{\text{th}}$ orbit is $r_n = a_0 n^2$ where $a_0$ is the Bohr radius.

Compare each option using $r_n \propto n^2$

Option A

$\frac{r_4}{r_2} = \frac{4^2}{2^2} = \frac{16}{4} = 4$ So, radius of 4th orbit is four times that of 2nd orbit.
A is correct.

Option B

$\frac{r_8}{r_4} = \frac{8^2}{4^2} = \frac{64}{16} = 4$ So, radius of 8th orbit is four times that of 4th orbit.
B is correct.

Option C

$\frac{r_6}{r_4} = \frac{6^2}{4^2} = \frac{36}{16} = \frac{9}{4}$ This is not equal to $3$.
So, radius of 6th orbit is not three times that of 4th orbit.
C is incorrect.

Option D

$\frac{r_3}{r_1} = \frac{3^2}{1^2} = 9$ So, radius of 3rd orbit is nine times that of 1st orbit.
D is correct.

Conclusion
The incorrect statement is: $\boxed{\text{C}}$