Answer
The slit separation should be decreased.
Work Step by Step
We can write an expression for $\theta$:
$d~sin~\theta = m~\lambda$
$sin~\theta = \frac{m~\lambda}{d}$
$sin~\theta = \frac{4~\lambda}{d}$
In order to eliminate all visible light in the fourth-order maximum, we need to increase the value of the expression $\frac{4~\lambda}{d}$ until it is greater than 1 for all wavelengths of visible light.
To increase the value of this expression, the slit separation $d$ should be decreased.
