I believe he's borrowing directly from Hindu mythology. The World Turtle mytheme is present in at least North American, Chinese, and Indian culture, while the World Elephant is more specific to Hindu mythology.

In terms of numbers, the elephants are at the compass points and so there are some multiple of four; for example in Ramayana the number is specified as exactly four:

deva daanava rakSobhiH pishaaca pataga uragaiH puujyamaanam mahaatejaa dishaa gajam apashyata

 

And he that resplendent Amshuman beheld one of the four directional elephant of the earth which is being venerated by gods, monsters, demons, imps, vultures and serpents.

While in Amarkosha (PDF) there are eight (as with lords and planets):

airāvataḥ puṇḍarīko vāmanaḥ kumudo 'ñjanaḥ puṣpadantaḥ sārvabhaumaḥ supratīkaśca diggajāḥ Respective elephants associated to the eight directions

In The Folklore of Discworld (co-written with Jacqueline Simpson), Pratchett discusses the origin:

Further details of Hindu cosmology vary. According to one myth, there are four (or eight) great elephants named the diggaja or diśāgaja, 'elephants of the directions', guarding the four (or eight) compass points of this disc, with a type of god called a lokapala riding on the back of each one. But the oldest texts do not claim that they carry the world. According to another myth, however, the world rests on the back of a single elephant, Maha-Padma, and he is standing on a tortoise named Chukwa. Finally, it is said in yet another muth that the god Vishnu once took on the form of a vast tortoise or turtle (kūrma), so huge that Mount Meru, the sacred central mountain of the world, could rest o fhis back and be used as a stick to churn the ocean. At some stage, though nobody knows just when, these insights began to blend, with the result that some (but not all) Hindu mythographers now say the world is a disc supported by four elephants supported by a turtle.

The book goes on to discuss the turtles all the way down idea, and the Chinese cosmic turtle (which contains the world), and the Thief of Baghdad's version:

ABU: Does the world have a roof?

 

GENIE: Of course! Supported by seven pillars, and the seven pillars are set on the shoulders of a genie whose strength is beyond thought, and the genie stands on an eagle, and the eagle on a bull, and the bull on a fish, and the fish swims in the sea of eternity.

I believe he's borrowing directly from Hindu mythology. The World Turtle mytheme is present in at least North American, Chinese, and Indian culture, while the World Elephant is more specific to Hindu mythology.

In terms of numbers, the elephants are at the compass points and so there are some multiple of four; for example in Ramayana the number is specified as exactly four:

deva daanava rakSobhiH pishaaca pataga uragaiH puujyamaanam mahaatejaa dishaa gajam apashyata

And he that resplendent Amshuman beheld one of the four directional elephant of the earth which is being venerated by gods, monsters, demons, imps, vultures and serpents.

While in Amarkosha (PDF) there are eight (as with lords and planets):

airāvataḥ puṇḍarīko vāmanaḥ kumudo 'ñjanaḥ puṣpadantaḥ sārvabhaumaḥ supratīkaśca diggajāḥ

Respective elephants associated to the eight directions

In The Folklore of Discworld (co-written with Jacqueline Simpson), Pratchett discusses the origin:

Further details of Hindu cosmology vary. According to one myth, there are four (or eight) great elephants named the diggaja or diśāgaja, 'elephants of the directions', guarding the four (or eight) compass points of this disc, with a type of god called a lokapala riding on the back of each one. But the oldest texts do not claim that they carry the world. According to another myth, however, the world rests on the back of a single elephant, Maha-Padma, and he is standing on a tortoise named Chukwa. Finally, it is said in yet another myth that the god Vishnu once took on the form of a vast tortoise or turtle (kūrma), so huge that Mount Meru, the sacred central mountain of the world, could rest on his back and be used as a stick to churn the ocean. At some stage, though nobody knows just when, these insights began to blend, with the result that some (but not all) Hindu mythographers now say the world is a disc supported by four elephants supported by a turtle.

The book goes on to discuss the turtles all the way down idea, and the Chinese cosmic turtle (which contains the world), and the Thief of Baghdad's version:

ABU: Does the world have a roof?

GENIE: Of course! Supported by seven pillars, and the seven pillars are set on the shoulders of a genie whose strength is beyond thought, and the genie stands on an eagle, and the eagle on a bull, and the bull on a fish, and the fish swims in the sea of eternity.