so, they have pocket 4s. when it comes to them, the pot has already like $60-70 in it, and they need to pay 15. if they hit their set, they will likely be ahead of everyone, and who knows how much they would be able to win from the rest of the people. generally, huge implied odds
- if it were heads-up, probably it wouldn't be worth it, but in a multi-way pot, set-mining like that is a must.
i would suggest you read up a little in the strategy and learning poker sections of the forum (the latter is not just for absolute beginners), and soon you will understand why people would make such a decision. good luck!