The Ivatt class 4 was a mixed-traffic locomotive designed for the London, Midland and Scottish Railway by their chief mechanical engineer of the same name. Categorized in power class 4, it was used for medium goods and secondary passenger trains. As it was only introduced in 1947, only three were delivered to the LMS. 159 more were built by 1968 directly for British Railways.

A feature were the highly-mounted, straight running boards which gave the locomotive a somewhat American look. Although this may have saved construction costs and facilitated maintenance, it was considered ugly by many British. The first 50 locomotives got double chimneys, these led to poor steaming and were later changed to a regular chimney.

Their main area of operation was the former LMS network. Due to their ungainly appearance in the eyes of the British, they got nicknames like “mucky ducks”, “doodlebugs” or even “flying pigs”. When they were used as the basis of the BR standard class 4 2-6-0, these were somewhat modified to match the aesthetic standards, for example with running boards which were now sloped at the front. The Ivatt locomotives were withdrawn between 1963 and 1968, so some lived longer than the BR standard locomotives.