It was chambered in the original 303 British. An obscure rimmed rifle round.
Obscure? I wouldn't call it that...Introduced in 1888 and officially the British service rifle cartridge until the late 1950s, it had a longer military service life than the .30-06 did.
And, as a sporting rifle round, filled the same niche throughout the British empire and Commonwealth that the .30-06 did in the US. Mostly is ex-service rifles sporterized to some extent though there were some fine rifles built as sporters from the design up chambered in .303 British.
And, btw, if the bolt head of your No4 Mk1 flips up on its own, the rifle is broken. There is a spring loaded catch on the action (and not particuarly easy to use on a new rifle) with the sole function of preventing the bolt head from flipping up, unless you push down on the catch, first. Unless this catch is missing or broken you have to use it to get the bolt out of the action.