Let electric field intensity at any point P which lie at a distance x m from 4Q be zero.

Electric field intensity $(E_1)$ due to $4Q$ at P

As direction of $E_1$ and $E_2$ are in opposite directions.

=> $\large\frac{1}{4 \pi \in _0} . \frac{4Q}{x^2} $

=>$ \large\frac{1}{4 \pi \in_0 }. \frac{Q}{(1-x)^2}$

=> $\bigg( \large\frac{1-x}{x} \bigg)^2 =\frac{1}{4}$

$\large\frac{1-x}{x} =\frac{1}{3}$

$\large\frac{1}{x} $$-1 =\frac{1}{2}$

$ \large\frac{1}{x} =\frac{3}{2}$

$x= \large\frac{2}{3} $$\;m$

Electric field intensity is zero at a point which lie at a distance $x= \large\frac{2}{3}$$m$ from $+4Q$ charge on the line joining two charges.