I may be wrong, but I thought at that time they still used five grooves. In the days of cut rifling they could get it smoother (or perhaps as smooth with fewer and deeper passes of the cutter) when the tool was backed up by a land opposite the groove.
It became quite pointless when they went over to broached rifling, as I believe all No4 rifles were. A very expensive tool had a long series of graduated teeth for each groove, and cut them all in one pass. The reason some No4 barrels (more, I think, than with earlier Lee-Enfields) have oversized groove diameter is probably that they started with the broaches oversized to permit the maximum number of resharpenings before they were discarded.
Savage-Stevens and Long Branch in Canada both produced two-groove barrels. Long Branch and possibly Savage started with five and changed to two. In fact Long Branch, after the war, used Bren gun equipment to make six-groove barrels with a right-hand twist. None of them were intrinsically better or worse than the others. It isn't true, as sometimes suggested, that two grooves speeded up the rifling process. Broaching was always equally speedy. It speed up toolmaking and resharpening.
I hear the US army was found to have warehouses full of Second World War underwear. I wouldn't be surprised if the British government has a room full of undersize broaches, just about right for a tight .308 barrel.