Following on from London Bus Routes One by One: 1-100, London Bus Routes One by One: 101-200 and London Bus Routes One by One: 201-300, this fourth volume in the series investigates those routes with the highest numbers used today. In times of old, the numbers 300-499 were restricted to the green buses of the old London Transport Country Area, with 200-399 covering the territory north of the river and 400-499 covering the territory to its south. Since the hiving-off of those operations, these numbers have been freed, and increasingly, they have been used to sectionalize longer routes, which have come up against London's relentless traffic and thus needed to be shortened again and again. Some, as might be expected, denote out-of-the-way minibus routes, but others, such as 390, work in busy central London. Though the higher-numbered routes have been around for less time overall, they too have been subject to the whims of tendering, with many regularly changing operators.

As in previous volumes, a potted history of each route is accompanied by routeing details and one or two pictures of the kind of bus that operates on that route. Illustrated with over 190 color photographs, this volume represents an up-to-date snapshot of the fascinating modern London bus scene as it stands in autumn 2021.