So first, we want to find how much in total did his elevation change. So we want to subtract the him starting from 6 feet below sea from the 98 feet below sea level, to get 92 feet.
Now, since he descended in four equal intervals, we can just divide 92 into four equal parts, or divide by 4, to get 23 feet.
So the diver's change in elevation in each interval was (negative?) 92 feet.
