The concentration of S4 is 50mg per ml, so 1ml (one full dropper) would be 50mg. I'm assuming you are doing 50mg per day which is normal recommended dose. If so, you would split that dose into two doses per day, meaning one 25mg dose (half a dropper 0.5ml) and the same thing later on.
What you have there shown is just 0.25ml which would be 12.5mg
25 was the Milligrams you should dose. The dropper is marked in milliliters. So you have to figure out how many ml you need based on the concentration. Since the bottle states 50mg/ml, and you want to dose half of that (25mg) then you would dose half of the ml (0.5ml)