After 4 days, we apply the second decrease to the amount after 2 days.
• After 2 days: 90 ml (given)
• After another 2 days: decreased by 30%
• Remaining = 100% − 30% = 70% of 90 ml
• 0.70 × 90 = 63 ml
Check: The first decrease (to 90 ml) is not needed for the 4-day answer. ✓
Final Answer: 63