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:

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

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:

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:

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.

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.