Answer: 1.75 cubic feet concrete is needed.
Explanation:
Alright, lets get started.
The volume of the pillar is :

We have given the height is 5 feet.
We have given the diameter is 4 inches.
So the radius will be :

Converting inches into feet.
The radius will be :

So the volume of the cylinder will be :

So the volume of the cylinder will be :

As there are 4 pillars, so total volume will be :

So, total volume will be :

Hence 1.75 cubic feet concrete is needed. : Answer
Hope it will help :)