In 1918 No.
46 was rebuilt by Lawson Billinton with a new boiler including a Robinson superheater.
Unfortunately the resulting locomotive was not tested before Billinton decided to rebuild other members of the class, using his K class superheated boiler.
The rebuilt locomotives were classified B4X. However, since the original frames could not be used - the K class firebox was too long to fit between the axles, new frames, new piston valve cylinders - they were virtually new engines.
Unfortunately the original motion and motion plate was retained - to save costs.
This meant that the piston valves were in the constricted space below the cylinders at an angle to the axis of the cylinders.
Only 8" diameter valves could be fitted in, and combined with the restricted exhaust arrangement ensured that they were unable to use the steam available from the excellent K class boiler.
Acceleration from stops was very leisurely and they could only be coaxed up to 70 mph (110 km/h), when newly outshopped, with great difficulty in places where the Atlantics, J class and Baltics could do 80 mph (130 km/h).
Twelve members of the class were 'rebuilt' between August 1922 and January 1924, but further  were deferred by Richard Maunsell of the Southern Railway when it became apparent that their performance was not satisfactory.
Harold Holcroft found, when he was tasked by Maunsell to report on the post grouping loco stock, that class B4x were very expensive compared to the SECR rebuilds (D1/E1) and far less competent.
It was to be 1929 before their services on express work could be dispensed with.
Henceforth the class was deployed on secondary duties.

The B4 and B4x classes continued with secondary duties but thirteen members of the class were withdrawn between 1934 and 1939.
The remainder would have followed soon after if the Second World War had not brought about a temporary reprieve.
Six B4s and twelve B4Xs passed to British Railways in 1948.
All had been withdrawn by 1951.
None has survived.
In 1918 No. 46 was rebuilt by Lawson Billinton with a new boiler including a Robinson superheater. Unfortunately the resulting locomotive was not tested before Billinton decided to rebuild other members of the class, using his K class superheated boiler. The rebuilt locomotives were classified B4X. However, since the original frames could not be used - the K class firebox was too long to fit between the axles, new frames, new piston valve cylinders - they were virtually new engines. Unfortunately the original motion and motion plate was retained - to save costs. This meant that the piston valves were in the constricted space below the cylinders at an angle to the axis of the cylinders. Only 8" diameter valves could be fitted in, and combined with the restricted exhaust arrangement ensured that they were unable to use the steam available from the excellent K class boiler. Acceleration from stops was very leisurely and they could only be coaxed up to 70 mph (110 km/h), when newly outshopped, with great difficulty in places where the Atlantics, J class and Baltics could do 80 mph (130 km/h). Twelve members of the class were 'rebuilt' between August 1922 and January 1924, but further  were deferred by Richard Maunsell of the Southern Railway when it became apparent that their performance was not satisfactory. Harold Holcroft found, when he was tasked by Maunsell to report on the post grouping loco stock, that class B4x were very expensive compared to the SECR rebuilds (D1/E1) and far less competent. It was to be 1929 before their services on express work could be dispensed with. Henceforth the class was deployed on secondary duties.
The B4 and B4x classes continued with secondary duties but thirteen members of the class were withdrawn between 1934 and 1939. The remainder would have followed soon after if the Second World War had not brought about a temporary reprieve. Six B4s and twelve B4Xs passed to British Railways in 1948. All had been withdrawn by 1951. None has survived.
LB&SCR B4 class