She has not been publicly identified.
The driver of the lorry was not injured and no arrests have been made.
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All eastbound lanes were closed for seven hours until 11.30am while police investigated.
Police are appealing for information the public may have about the collision.
“I am appealing to anybody who was driving on the M4 at around the time of the collision to please check their dash-cam footage and contact Thames Valley Police if it has captured anything that can assist this investigation,” said the senior investigating officer, Sergeant Matt Cadmore.
“I would also urge anybody who saw the female on the carriageway in the moments leading up to the collision, or who witnessed the incident, to please get in touch.”
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The woman was reportedly seen at walking down the eastbound carriageway at 4.15am towards London.
The collision caused hours of delays for drivers, with queues backed up for five miles.
Witnesses can report to police by calling 101or online, quoting incident reference number 0200 of January 27.