Father-of-two Mark Pilgrim, 48, from Tilehurst, was struck on the eastbound carriageway between Reading and Maidenhead on Friday night.
His death is being treated as murder by Thames Valley Police.
Det Ch Supt Chris Ward said he believed Mr Pilgrim was "ejected" from the vehicle he was travelling in a mile east of Junction 10, towards London.
He added: "We know that at about half past four in the afternoon he was on a bus in Reading but I need to know who he was with, what he was doing and who he was talking with."
A female driver reported hitting a man on the motorway between junctions 10 and 8/9 at about 22:00 BST on Friday. He was declared dead at the scene by paramedics.
A post-mortem examination held on Sunday found Mr Pilgrim had multiple injuries to his head and body, consistent with hitting the ground from a vehicle.
Det Ch Supt Ward said: "Our work so far suggests that Mark was alive until he left the vehicle on the M4 and so we want to hear from anyone who saw anything suspicious on that stretch of motorway before 10pm on Friday.
"We are also trying to trace Mark's final movements on the day of his death, so if you saw Mark on Friday, please get in touch."
A man who was hit by a car on the M4 in Berkshire had been thrown from a moving vehicle, police believe.