No.4 Loch was one of two locomotives delivered for the opening of the south line on 2nd  August 1874, the other being No.5 Mona.  Named after the then-lieutenant governor of the island Henry Brougham Loch (the first Baron Loch), it was fitted with a new boiler in 1893  and again in 1909 at which time it was also rebuilt as a medium-boilered locomotive improving the tractive effort to that akin to younger sisters No.10 G.H.Wood and No.11 Maitland.  Another boiler was fitted in 1946, this having been originally fitted to No.5 Mona in 1914, which saw the locomotive remain in traffic until 1955 after which is was withdraw and stored in the carriage shed at Douglas Station for several years.

Looking to invest in the future of the railway upon taking it over in 1967, Lord Ailsa identified the locomotive as being a candidate for a new boiler in 1967 and work began stripping it down; in early 1968 a boiler was delivered from the Hunslet Engine Co., and work began assembling the locomotive for service and steam tests were carried out.   However at the end of September 1968 the railway closed entirely which gave No.4 the odd distinction of being the first locomotive ever to be commissioned on the day the lines were closed.  When services resumed in 1969 only the south operated and No.4 became a regular feature, latterly based regularly at Port Erin. 

Upon nationalisation of the railway a policy of different liveries was adopted and the locomotive was outshopped in a new Royal Maroon scheme, emerging for a press event to launch the season.  It also featured a painted-on . on the tanks which was later replaced with  replica worksplates on the cab sides.  It was also the only locomotive to carry a non-functioning bell-mouth dome (until the Salter safety valves were reinstated in 2021).  It was known at this time for wearing a triskelion in brass on the buffer beam with a 4 numeral.  In 1992 it performed trials on the electric railway in preparation for the Year Of Railways when it hauled special Steam Under The Wires trains to Dhoon Quarry reflecting the fact that No.2 Derby had been used in 1898 to construct parts of the line; this was repeated in the following year, the locomotive was notably returned to home metals chimney-first from Port Erin at this time to add photographic variety.  It was withdrawn after the 1995 Santa Trains and placed once again in storage. 

Following withdrawal the Un-Loch Your Cash appeal was launched by the Supporters’ Association (1998-2000) which ultimately funded a replacement boiler and saw a return to service in 2002 and regular operation, outshopped in the Indian Red livery.  The boiler certificate expired in August 2015 and another overhaul was commenced in 2017 which saw major rebuilding of all moving parts replacement side tanks, cab floor, bunker and replacement pipework.  Boiler work was overhauled by the Severn Valley Railway including a new inner firebox and the decision made to return the Salter safety valves which have not been seen in service on the railway for over six decades.  In early 2021 she had several test runs and was returned to service when the season commenced in late May.  For the first season the brassware on the buffer beams was not replaced to give a true 1950s appearance, these were put back on in 2022.  Today the locomotive remains part of the regular service fleet.