Explanation:

- Initially, the milk to water ratio is 4:1.

- Let’s assume there are 4 liters of milk and 1 liter of water.

- Total mixture: 5 liters with 4 liters milk and 1 liter water.

- To make quantities equal, add enough water so both are the same (4 liters each).

- Add 3 liters of water to make it 4 liters.

- New total of water = 4 liters.

- Percentage increase = (added water/original water) * 100 = (3/8) * 100.

- Calculate this percentage: 37.5%.

The correct answer is Option: 4, 37.5%.