Krieger and Lothar Walther have made replacement No. 4 barrels, but in .303.
Unless you can do the work yourself, this will not be an inexpensive project. A nice LB No. 4 .303 will bring $1200 or more. A DCRA conversion in unaltered condition perhaps $2000. A really sound No. 4 action needs to be used - the 7.62x51 and .308 are pretty much the limit of what a No. 4 can handle. A sported LB could be the starting point, but No. 4 stock sets with fittings are now pricey.
The 7.62x51 conversions were done in the 60s. It was soon observed that it was difficult to get a converted service configuration No.4 to shoot as well as a comparable .303. After the rules changed and heavier barreled target rifles were being made on No. 4 actions, they were popular for longer ranges with service ball for about 20 years. Once target .308 ammunition was approved, the Lee Enfield rifles disappeared from competition.
Just a suggestion... If you can find a 26" .308 take-off barrel, it could be reworked for a No. 4 project. Remington did make varmint weight barrels this length. There are also take-off target rifle barrels. The breech can be altered for the No. 4 receiver, overall length trimmed to 25 1/4", and the contour adjusted as required. This might be the least expensive way to set up a 7.62x51 No. 4.
Years ago I reworked a take-off barrel from a target rifle for installation on a No. 4. Looked normal externally, shot pretty well.
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