4. (8 points) There is a magical tree with 123 fruits on it. On the first day, 1 fruit will fall from the tree. Starting from the second day, the number of fruits that fall each day is 1 more than the previous day. However, if the number of fruits on the tree is less than the number that should fall on a certain day, then on that day it will start over by dropping 1 fruit, and continue according to the rule, and so on. So, on which day will all the fruits on the tree have fallen?
17
【Answer】Solution: Since $1+2+3+4+5+6+7+8+9+10+11+12+13+14+15=120$, by the sixteenth day, there are not enough 16 to start over. $1+2=3$ that is $1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+1+2=123$ (items). Therefore, the answer is: 17 days.