1 is 3, 2 is 3, 3 is 4, 4 is 4 and 5 is 4.

How can this possible?(explain the
rule)

To explain the rule behind the given sequence:

The rule seems to be that each number corresponds to the number of letters in its English spelling.
Let's break it down:

1 is 3: The word "one" has three letters.
2 is 3: The word "two" has three letters.
3 is 4: The word "three" has four letters.
4 is 4: The word "four" has four letters.
5 is 4: The word "five" has four letters.

Therefore, the rule is based on counting the number of letters in the English word representing each number.