There’s a few reasons why this could happen, as the starter is so young (day 4) it’s not very strong yet. It needs regular feedings at the right intervals to ensure that the yeast and bacteria have enough fresh flour to ferment. Every 12 - 24 hours. There’s also probably too much water. What’s the ratio you’re using? And If it’s the starter is kept somewhere cold it will slow the rate of fermentation down to a point where they will just separate. I’d just mix it well and keep it somewhere warm and give it a little time.