In modern periodic table there are 18 vertical columns called groups and 7 horizontal rows called periods.
As per question it is asking about the least metallic element
The element of 4th period are indicated as below
So as we move from left to right along the period i.e from K ( Potassium ) to Br ( Bromine )
atomic or ionic radii decreases due to increase in effective nuclear charge.
On the basis of this Br has smallest size in its period and thus it can not loose electron easily with respect to its period.
We know that element which can easily loose electron are metallic and thus on the basis of this we can concluded that Bromine is the least metallic in its respective period.
Here we are not taking noble gases i.e group 18 elements.