This is a tough one...the fourth act of mercy is Ransoming the Captives.
I have to admit to not knowing a single person in captivity that would need financial ransoming, which I am assuming was the original meaning of this act of mercy. (I could be wrong though..it's happened before..) However, there are others ways in which people are held captive.
The first could simply be someone who is an invalid. They are being held captive by their physical or psychological infirmities. A visitation, a phone call or a card, anything that could relieve them, even for just a little while from their "captor" certainly would fulfill this act of mercy.
The second requires a little more legwork. Believe it or not, the child sex trafficking business is a horrendous problem that plagues many of America's cities. According to Streetlight PHX, a group dedicated to eradicating child sex slavery in Phoenix, Arizona, the average age in which a child enters prostitution is 13. Unbelievable. Finding an agency such as Streetlight PHX in your area and supporting it in any way you can, even if it's a couple bucks just once would certainly help ransom some poor kid stuck in such a desperate situation.