The GWR 4100 Class was a class of steam locomotives in the Great Western Railway (GWR) of the United Kingdom.

The first Locomotives of the Great Western Railway (GWR) were specified by Isambard Kingdom Brunel but Daniel Gooch was soon appointed as the railway's Locomotive Superintendent. He designed several different 7 ft 1⁄4 in broad gauge types for the growing railway, such as the Firefly and later Iron Duke Class 2-2-2s. In 1864 Gooch was succeeded by Joseph Armstrong who brought his standard gauge experience to the workshops at Swindon. To replace some of the earlier locomotives, he put broad gauge wheels on his standard gauge locomotives and from this time on all locomotives were given numbers, including the broad gauge ones that had previously carried just names.