This just needs a roll, it's the fat from your milk settled. Along as in the 4 days I would say it's fine. Xx
If it smells ok then it's fine. I follow the 6-6-6 rule, 6 hours at room temp, 6 days in fridge, 6 months in freezer. It's just the fat on top. I've got one in the fridge that looks the exact same! Give it a good shake and it will be fine
By the 4th day milk is bad remember u can use it for baths or other things but before u fill again make sure you stererlise again