Reddit reviews The Fifth Elephant: A Novel of Discworld

And four elephants on that turtle, with the Earth on their back.

Legends have it that there was once a fifth elephant, but he fell off.

Recommended reading for anyone wanting to know more:
https://www.amazon.com/Fifth-Elephant-Novel-Discworld/dp/0062280139/ref=mp_s_a_1_1?ie=UTF8&qid=1535944541&sr=8-1&pi=AC_SX236_SY340_FMwebp_QL65&keywords=the+fifth+elephant+terry+pratchett&dpPl=1&dpID=51yuxTs%2Bm-L&ref=plSrch

They're talking about the relationship between Leonard of Quirm and Lord Vetinari.

These two feature in quite a few of the Discworld novels, but chiefly it would be in:

Wyrd Sisters
Men at Arms
Jingo
The Fifth Elephant
The Last Hero

All Amazon-linked for your convenience. ;)