Named after lieutenant governor Henry Brougham Loch and beloved by many as their "favourite" engine on the railway, As originally built "Loch" was a small boilered locomotive, but she was rebuilt as a medium boilered locomotive in 1909. This gave her the same tractive effort as Nos 10 and 11. No. 4 has the strange distinction of being what must surely be the first locomotive ever to (re)enter service on the day the line closed. Having been earmarked for re-boilering in 1967 by the 1968 season she was ready for service and steam tests were carried out accordingly. Fate intervened however and at the end of September 1968 the Peel and Ramsey lines closed for good. Fortunately, the Port Erin line was retained and No. 4 was familiar to many as the south based engine for many years, right up until her withdrawal from service after the 1995 Christmas services on the railway. She is the only locomotive to carry a non functioning "bell-mouth" dome and a non-standard livery of maroon was carried from 1979 to withdrawal. Of course, now she wears the familiar Indian red but is distinguished from her shed-mates by carrying a Legs Of Mann and "4" numeral on her buffer beam. She was used heavily in the marketing campaign for the 1993 "Year Of Railways" when she was the locomotive chosen to haul special services on the Manx Electric Railway. Following the Un-Loch Your Cash appeal by the Isle of Man Steam Railway Supporters' Association in 1998–2000, she returned to service in 2002 and was a regular fleet member once more. On 20 May 2008, she collided with a van and badly damaged her buffer beam. The boiler certificate expired on 31 August 2015 and following that day's services the locomotive with withdrawn from traffic; it is not known when it is likely to return.
Loch was seen in Hashire! The "Thomas the Tank Engine and Friends" Steam Locomotive is Alive!, where where host Gaku Hamada is riding while learning about how the railway works.