The correct answer seems to be number four - not enough. It both suits grammatically, and sentence-wisely.
1.It is not wise to trust strangers, even when they promise to be loyal to you.
2.Ignoring the wisdom and experience of others can lead to terrible consequences.
3.You should never give up, even if you think a problem is impossible to solve.
4.You can overcome tragedy if you rely on your family and friends.
