The A4 is a major road in England from Central London to Avonmouth via Heathrow Airport, Reading, Bath and Bristol. It is historically known as the Bath Road with newer sections including the Great West Road and Portway. The road was once the main route from London to Bath, Bristol and the west of England and formed, after the A40, the second main western artery from London.

Although most traffic is carried by the M4 motorway today, the A4 still acts as the main route from Bristol to London for non-motorway traffic.

The A4 has gone through many transformations through the ages from pre-Roman routes, Roman roads (such as the one passing Silbury Hill), and basic wagon tracks. During the Middle Ages, most byways and tracks served to connect villages with their nearest market town. A survey of Savernake Forest near Hungerford in 1228 mentions "The King's Street" running between the town and Marlborough. This street corresponded roughly with the route of the modern A4. In 1632, Thomas Witherings was appointed Postmaster of Foreign Mails by Charles I. Three years later, the king charged him with building six "Great Roads" to aid in the delivery of the post, of which the Great West Road was one.

It was not until the 17th century that a distinct route between London and Bristol started to resemble today's road. During the 17th century, the A4 was known as the Great Road to Bristol. When Queen Anne started patronising the spa city of Bath, the road became more commonly known as Bath Road. Over the years, the direction of the road has taken many detours depending on such factors as changes in tolls or turnpike patronage. For example, in 1750 the toll road from London was altered to go through Melksham; and in 1695 the map maker, John Ogilby, produced a map of the Hungerford area of the Great West Road showing two possible routes.

As Bath became more popular with the wealthy and famous, it was inevitable that turnpike trusts would be set up under the terms of the Turnpike Acts to pay for maintenance and improvements to the road. The first turnpike on this road was between Reading and Theale in 1714. Due to increasing traffic, sections of the road between Kensington, over Hounslow Hill, to Twyford were turnpike by 1717 with the remaining sections placed under turnpike trusts.

As turnpike trusts were individually run, there was the possibility for greatly differing road conditions, especially over the London Clay basin of Kensington, Brentford, Hounslow and Slough, where winter conditions left the way muddy and uneven. This was not always the case with the Bath Road, as many of the wealthy landowners along the route co-operated informally and exercised a large amount of control over feeder roads. As a result, control of the Bath Road was easy to maintain and many inns and towns became prosperous.

Tollhouses were established at Colnbrook, Maidenhead, Twyford, Castle Street Reading, Thatcham and Benham. During the 1820s, the employment of good surveyors improved the condition of the road and aided an increased flow of wealthy travellers. The tolls raised from such clientele ensured that when the turnpike trusts handed over the route to local highway boards, they had no financial liabilities. Justices of the peace were empowered by the Highway Act 1862 to combine turnpike trusts into highways districts. This meant that by the late 1860s trusts were either not renewing their powers or were being terminated by general Acts of Parliament. For example, most turnpikes in Berkshire, including the Bath Road, were officially wound up by 1878 when legislation transferred responsibility for dis-enturnpiked roads to the new county councils.^{[dubious – discuss]} The tollgate on the Bath Road west of Reading was removed in 1864 as the outward pressure of urban development made rates a more acceptable way of financing the maintenance of what was now a suburban road.

With the improvement being made to the road systems, the business of moving mail became easier and thus more profitable as volumes were able to increase. In Bristol, a postal office had been well established by the 1670s. The journey time to London at this period was about 16 and three quarter hours. A letter from Bath in 1684 took about 3 days going via a postal office in Marshfield on the Bristol Road. (The route to Bristol did not yet go through Bath at this time). Journey times during the Turnpike era fell with the improvements from 2 days in 1752 to 38 hours in 1782 and 18 hours by 1836. Royal Mail coaches in 1836 were able to do the trip in 12 to 13 hours.