The frequency of 1st line of Balmer series in H2 atom is v0. The frequency of line emitted by singly ionised He atom is

2$v_0$ 4$v_0$ $v_0$/2 $v_0$/4

4$v_0$

$\nu \propto Z^2 \Rightarrow \frac{\nu_{H_2}}{\nu_{H e}}=\left(\frac{1}{2}\right)^2=\frac{1}{4}$ $ \Rightarrow \nu_{H e}=4 \nu_{H_2}=4 \nu_0$