Since we are trying to find the probability for a sequence of events then we need to multiply the probability of each event happening separately from each other. Since the probability of it raining on a single day is 2/3 we need to multiply this value with itself 4 times (one for each day).
2/3 * 2/3 * 2/3 * 2/3 = 16/81 or 0.1975 or 19.75%
Therefore the probability would be 16/81 or 0.1975 or 19.75%
