We don't often look at four words that can be easily confused for each other, but this pack is an exception. Let's start with our base word: pity. Pity is to feel sorrow or compassion for someone's misfortunes or sorrows. It can also be something that causes sorrow or disappointment:

Pity the misunderstood conifer: Evergreens' many benefits overlooked

Pitiless, then, is without (-less) pity, lacking compassion. Pitiless can be cruel or merciless:

Hell, on the other hand, will be like having to read an unauthorised biography of yourself written by a pitiless researcher who has exposed your secrets.

Pitiable, piteous, and pitiful all mean some shade of deserving pity. Let's see if we can untangle the differences. Piteous and pitiable both mean being deserving of pity:

A single drought could spell the end of a society and doom its inhabitants to piteous deaths.

Barb tried to comfort me as Rusty looked on, giving me a pitiable shake of the head.

Pitiable is found 10 times as often, however, as piteous in Google News search results. Because of this difference, piteous has become a more archaic, poetic word, while pitiable remains more common. The latter also means scornfully small or poor:

All of these foods proffer very pitiable nutritional value, and it consequently makes no sense at all to carry on eating them.

Which brings us to pitiful. It, too, means deserving of pity and carries that second meaning of scornfully small or poor. It's used, though, in a much more negative sense, giving rise to feelings of contempt rather than compassion:

They are asking what is this pitiful circus financed with millions of their tax dollars.

Maine Gubernatorial Coverage: Sloppy, Pointless, Pitiful

Union chief endorses 'pitiful' bonus deal for Palm Beach County

If you want to say, then, that something deserves pity, pitiable is your best choice. If you're looking for a more poetic phrasing, go with piteous. A contemptuous meaning wants pitiful. And if you're all out of pity, go with pitiless.