The correct phrase is "among three boys" because "among" is used when referring to a group or collection of people or things. In contrast, "between" is used when referring to two people or things, not three.
- To: Is used for direction or purpose, not in the context of dividing things between people.
- For: Generally used to indicate purpose or intended recipient, not for distribution.
- Between: Used for two people or things.
- Among: Correctly used for dividing items among a group of people.
Thus, the correct answer is "among".
\[
\boxed{4. \, among}
\]