It wasn't actually in US use.Found thishttp://en.wikipedia.org/wiki/British_military_rifles#Rifle.2C_Number_4During the Second World War, the British government also contracted with the Canadian and US manufacturers (notably Long Branch and Savage) to produce the No. 4 Mk I* rifle. US-manufactured rifles supplied under the Lend Lease program were marked U.S. PROPERTY on the left side of the receiver.Yours is clearly marked on the left side of the receiver, (1st picture last post) No 4 Mk1*
They were probably used as training rifles by the US which would explain the markings.An excerpt from wiki..During the course of the Second World War, the No. 4 rifle was further simplified for mass-production with the creation of the No. 4 Mk I* in 1942, with the bolt release catch replaced by a simpler notch on the bolt track of the rifle's receiver. It was produced only in North America, by Long Branch Arsenal in Canada and Savage-Stevens Firearms in the USA
I posted that same thing four hours ago...sort of!
True, but the listing you found did not explain the "US PROPERTY" marking so I figured it was worth posting.
Remember Timothy, The Google-Fu is strong with the Bogan....