Since $ξ$ follows a normal distribution $N(\mu, \sigma^2)$, and given $P(ξ 6) = 0.15$, it implies that $2$ and $6$ are symmetrical about the mean $\mu$ of the normal distribution. Therefore, the mean $\mu = \frac{2 + 6}{2} = 4$.

The probability that $ξ$ is between $2$ and $4$ can be calculated as follows:

Since $P(ξ 6) = 0.15$, the probability that $ξ$ is between $2$ and $6$ is $1 - 0.15 - 0.15 = 0.7$. Given the symmetry of the normal distribution about its mean, the probability that $ξ$ is between $2$ and $4$ (which is half of the interval from $2$ to $6$) is half of $0.7$, which is $0.35$.

Therefore, the correct answer is $\boxed{\text{B: } 0.35}$.