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Literary Sources of the Buddha Legend says,
In the early Pali sources, the legendary account of the four sights is only described with respect to a previous legendary Buddha Vipassī (Mahāpadāna Sutta, DN 14).[13] In the later works Nidanakatha, Buddhavamsa and the Lalitavistara Sūtra, the account was consequently also applied to Siddhārtha Gautama.
The "Mahāpadāna Sutta, DN 14" isn't one of the suttas translated in the Digha Nikaya section of accesstoinsight.
There is an English-language translation of it here: http://tipitaka.wikia.com/wiki/Maha-padana_Sutta
I don't know what current scholarship says. This paper from 1914 starts with,
To students of Buddhism and Comparative Religion desirous of knowing Buddha's own views and teaching from his own words, it is extremely disconcerting to find that the Pali Canon can no longer be regarded as the actual "Word" and Doctrine of Buddha himself.
... and goes on to claim that the doctrine that there were previous Buddhas was invented after the historical Buddha's death,
The theory that former human Buddhas preceded Gotama, although generally accepted as an integral part of Buddha's Buddhism, seems to me to have been invented after the Buddha's death.
... which would imply that DN 14 were a later invention.
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I'm not sure when the story of the four sights was first mentioned but it definitely doesn't seem to be a part of the early texts. The Devaduta Sutta mentions that seeing a baby, old person, sick person, a person punished, and a corpse as all being a kind of message sent to humanity to be reflected on, so perhaps some of this was interpolated into the biography.
Personally I don't think the traditional biographies have much historical merit. The earliest texts we have don't mention a lot of these kinds of things, and a lot of the embellishments are also found in the biographies of Mahavira, the founder of Jainism so I think there was probably a lot of borrowing of folk traditions. I think that these stories are very useful as a source of moral inspiration, but I think that they are very weak as a source of actual history.
Upvote:2
Possibly a later development based on AN 3.39. In that sutta, the Buddha didn't explicitly mention He saw those signs but the ideas were there:
...Amid such splendor and a delicate life, it occurred to me: ‘An uninstructed worldling, though himself subject to old age, not exempt from old age, feels repelled, humiliated, and disgusted when he sees another who is old, overlooking his own situation. Now I too am subject to old age and am not exempt from old age. Such being the case, if I were to feel repelled, humiliated, and disgusted when seeing another who is old, that would not be proper for me.’ When I reflected thus, my intoxication with youth was completely abandoned...
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On my memory, it first appears in the Buddhacarita, the earliest "full biography" of the Buddha, written by Asvaghosha around 100-150 CE.