Are you sure the timer was set to 30 minutes ? Not 30 x 0.1 hour ? (= 3 hours)
The unit is usually deci-hours ( = 0.1 hour)
Increasing from 3 hours to 4 hours will decrease your SD Traffic. There is less “SD requests”. However, the SD drops may be as numerous as before, because the SD drops are more often linked to Call setup or SMS, or Location Area update due to LAC change (not periodic update).
SD drop / SD request ==> SD drop rate increases.