4 In a certain mountainous region in winter, the probability of more than 20 cm of snow falling on any particular day is 0.21 .
- Find the probability that, in any 7-day period in winter, fewer than 5 days have more than 20 cm of snow falling.
- For 4 randomly chosen 7-day periods in winter, find the probability that exactly 3 of these periods will have at least 1 day with more than 20 cm of snow falling.