In order to calculate the yard line at the end of the fourth down we would have to make the following calculations:
On the next three downs they lost 6 yards, so if they lost we would have to substract from 25 yard the 6 yards, so first substract 25-6=19
They gained 10 yards, so 19 +10=29
Finally they lost 8 yards, so 29-8=21
Therefore, the yard line at the end of the fourth down was 21