4. Buratino buried two ingots on the Field of Wonders: a gold one and a silver one. On the days when the weather was good, the gold ingot increased by $30 \%$, and the silver one by $20 \%$. On the days when the weather was bad, the gold ingot decreased by $30 \%$, and the silver one by $20 \%$. After a week, it turned out that one of the ingots had increased, while the other had decreased. How many days was the weather good?
Solution. Increasing a number by $20 \%$ is equivalent to multiplying it by 1.2, and decreasing a number by $20 \%$ is equivalent to multiplying it by 0.8 (for $30 \%$ - by 1.3 and 0.7, respectively). Therefore, the result does not depend on the sequence of good and bad weather days, but only on the number of good and bad days.

After one good and one bad day, both ingots decrease: $1.2 \cdot 0.81$, while $1.3^{4} \cdot 0.7^{3}<1$. On the other hand, after two good and one bad day, the gold ingot increases. Therefore, it also increases after four good and two bad days, and even more so after five good and two bad days. Thus, if there are five or more good days, the gold ingot grows. Therefore, only when there are four good days does one ingot grow while the other decreases. Answer: four.

Comment. If the fact that the answer does not depend on the sequence of good and bad days is used without justification, no more than 4 points are awarded.