Standard class 5 and 4 comparison

Based on experience gained with the LMS class five or 'Black Five' mixed traffic design of Stanier Riddles also improved the stock and extended it with a lesser powered class 4 mixed traffic design.

The design work was done at the ex-LNER Doncaster Works but the bulk of the construction was done at Derby Works. The locomotive featured a Type 3B boiler almost identical to that of the Black Fives. A higher running plate, slightly enlarged driving wheels (from 6 ft 0 in (1.829 m) to 6 ft 2 in (1.880 m)) and a standard cab were the most obvious visible changes.
The first of the class, 73000, was outshopped from Derby in April 1951 and 30 were in service by January 1952. There was then a gap in construction before Derby resumed building its remaining 100 engines. 42 were built at Doncaster, starting in August 1955 and finishing in May 1957, with Derby's last engine following a month later.
Thirty engines, numbers 73125 to 73154, were built with Caprotti valve gear and poppet valves. One of these engines (73129) survives today at Butterley, and along with BR Standard Class 8 are the only engines with Caprotti gear left in Britain. (Wikepedia)

The class 4 was introduced in 1951. They were designed for mixed traffic use on secondary routes where the otherwise ubiquitous BR standard class 5 and their predecessors, the Black Fives, would be too heavy. They were essentially a tender version of the standard 4 2-6-4T, with similar characteristics to the GWR Manor Class, though unlike the Manors they were built to the universal loading gauge. They used the same running gear as the tank engine (with the leading bogie from the Standard Class 5), and substantially the same firebox, smokebox and boiler, although the boiler barrel was increased in length by 9 inches (23 cm).
Design work was done at Brighton by R. A. Riddles, with help from Swindon, Derby and Doncaster. Construction was at the BR Swindon Works.
The engine weighed 67.90 long tons (68.99 t), was 60 ft 0 in (18.29 m) long, with 5 ft 8 in (1.727 m) diameter driving wheels. It had two cylinders of 18 inches (457 mm) diameter and 28 inches (711 mm) stroke operated at maximum boiler pressure of 225 lbf/in² (1.55 MPa), to produce 25,515 lbf (113.5 kN) tractive effort. Its British Railways power classification was 4MT.
It normally used the standard BR2 or BR2A tender, which weighed 42.15 long tons (42.83 t) and carried 3,500 imp gal (16,000 l; 4,200 US gal) of water and 6.00 long tons (6.10 t) of coal. In this configuration its route availability was 4, almost universal over the British Railways network. (Wikepedia)