|From:||Westway, Marylebone, London|
[[Image:Motorway Left.svg|[ |x30px|link=]] M25 motorway
[[Image:Motorway Left.svg|[ |x30px|link=]] M40 motorway
[[Image:Motorway Left.svg|[ |x30px|link=]] M4 motorway
|Wembley, Harrow, Northwood, Rickmansworth, Amersham, High Wycombe, Marlow|
The road initially follows a course through north-west London via Harlesden, Wembley, Harrow, Northwood and Rickmansworth. The M25 Junction 18 is at Chorleywood. The A404 crosses into Buckinghamshire and continues towards Little Chalfont and Amersham.
Between Paddington and Amersham, the road closely follows the route of the London/Harrow-on-the-Hill/Aylesbury railway lines, The Chiltern Line, from Marylebone, and runs near several stations along that line.
At Amersham Common, the road turns south-west and continues in that direction joining the Amersham by-pass (A413) for a short distance, and then proceeds towards High Wycombe. After passing through the town centre, and crossing the A40, it changes to a dual carriageway up the hill to the M40 Junction 4, and continues as a dual carriageway past Marlow and Bisham.
A number of changes have been made to the course of the A404 since the 1970s including:
- The building of the "Marlow by-pass" section of the A404 in 1972 (which replaced the old road from Handy Cross to Marlow).
- The building of the "Amersham by-pass", A413, in 1987 which meant the A404 no longer went through Amersham Old Town, but skirted to the west and north of Amersham.
- The building in 1992 of the section by-passing Burchetts Green roundabout and the village of Stubbings in Berkshire. This section also allowed the A404 to continue neatly into the A404(M) (formerly the A423(M)) without drivers having to negotiate a roundabout.