In message , Richard J.
writes
- In the early 1960s, the Maidenhead and Slough bypasses were built. I
thought they were always M4, but perhaps one or both were A4(M) for a
time. Certainly from about 1963 onwards, the M4 was planned as a
London-South Wales route.

At the time, the route was to be north of Reading, and what's now the
A404(M) was the last few miles of the M4 (up to the A4 at Burchetts
Green). There were some old M4 signs on that road up until quite
recently (not checked it for a year or two).
--
Roland Perry