Currently available way to get a 4 round magazine Winchester 70 375 H&H

[Ontario Hunter]

Well, I misstated the obvious: If we are talking about Oberndorf bottom metal, then I assumed (we know how that goes), that a reader would understand the mechanism, with the hinged floorplate that opens and swings down.

The point is Pre-64 use an integral magazine box, properly dimensioned for the cartridge(s), and the feed rails of the action correspondently dimensioned for proper feeding (a la M98).
Post 1994 Classics use a separate magazine box, that dimensions determine the feeding for the cartridge(s).

Interesting about the NECG Magnum follower and trimming to work with .404 Jeffery.
I bought one for an Interarms Whitworth .375 H&H, that was not feeding properly. That and a new magazine spring, problem solved.

I tried several places to find a follower spring that was not tapered at the end like military Mausers. No luck. NECG used to sell magnum springs but no longer when I needed one. I seem to recall Hoosier or Midway also listed them but out of stock. My follower from NECG would have been a good fit for magnum Mauser action but lengthwise a tight fit in my standard action build for 404. The very slight trimming to follower side tab(s) was mostly to fix occasional hangup where mag box meets receiver. Mostly, as I recall, it involved beveling the square edges of tabs.