In 1912 N°59 was fitted with a Phoenix superheater, a rather large piece of apparatus that required much modification of the engine. A vast extended smokebox was fitted and the frames were modified to the extent that all the elegant curves were lost, being replaced by angle irons and an upward curve at each end. The experiment was not a success (as various other railways also discovered) and did not last long. Then, in 1935, the Southern Railway combined parts of N°59 (including the modified frames) with parts of N°68 to form composite engine N°2068.

Some examples were named after events and people involved in the Boer War whilst two, N°42 His Majesty and N°46 Prince of Wales, were frequently used for Royal Trains, and as such kept their names after Marsh scrapped nearly all of them until finally seeing them removed by the Southern Railway. Empress and Sirdar claimed a place in the history books when they worked Queen Victoria's funeral train which, on being taken over from the L&SWR at Fareham running late, arrived at Victoria some two minutes early. Empress was the train engine whilst Sirdar was the pilot engine running ahead of the funeral train.

N°52 Siemens was, briefly, renumbered 50 by mistake in 1905 and in 1908 was re-named Sussex (the name of N°72) due, it is believed, to an error in the paint shop! In addition to the two locos mentioned above, Richmond, Sussex (N°72), Princess Royal, Norfolk, Billinton (was Balmoral) and Devonshire kept their names until Southern Railway days.

N°74 was withdrawn during 1935 by the Southern Railway, but was returned to traffic in 1937 and subsequently passed into British Railways' stock in 1948. N°47, N°50 and N°60 were fitted with extended smokeboxes in 1915, N°44 was rebuilt with a superheater in 1922, N°45 was originally fitted with a Drummond water-tube firebox which was removed in 1911 and N°46 was rebuilt with larger cylinders and extended smokebox in 1918.

In 1922 Lawson Billinton started the rebuilding of twelve of the B4 class into class B4x, though in reality they were more like new engines as little of the "donor" B4 was left other than the wheel centres and bogies. The wheelbase was extended by some 2' 4" and the larger boiler pitched some 8' 10" above rail level. A superheater was added and the tender enlarged to hold an extra 600 gallons of water. The boiler was similar to that fitted to the K class with top feed, though not in the additional dome that was fitted to some classes. The first two engines, N°55 and N°60, were turned out in grey and remained so until painted green by the Southern Railway. The last ten B4xs were built by the Southern Railway.