Richard Macer-Wright has found some early information about the Maidenhead Bypass:
Just wondered whether you had seen a clip from British Pathe - it rather suggests the Maidenhead bypass opened on the ground as the M4 and was only known by A4(M) in atlases
Andy Syms answers just why this bit was abandoned:
I lived in Reading in the 50's, 60's and 70's and my recollection was that the 'Maidenhead bypass' was always designated the M4.
The reason that a withered arm was left when the M4 extended westwards was due to a change of route. Originally the M4 was due to pass to the north of Reading but the 'posh' inhabitants of Sonning and Caversham raised merry hell about it and so the route was altered to pass to the south. It passes within a quarter of a mile where I used to live in Shinfield but fortunately I'd moved by then...