Keep in mind that Jack O'Connor did much of his shooting with a fixed 4X that was horrible by todays standards. We tend to over scope.
Not sure what make of scope O'Connor used so having recently acquired one of his books delved into it to find out. For his sheep hunts it seems he used either a Redfield Bear Cub 4x or a Leupold 4x scope. This was to keep weight down more than anything as I read it. Now I have never encountered a Bear Cub scope so cannot comment on their optics but I do own a Leupold M8 Compact 4x scope, not sure of vintage, but the optics are clear enough if a little on the light side of 4x mag that is claimed when compared to a 60's vintage Khales 4L2 scope or the Pecar 4x36 Champion I also have.
Of similar vintage to the O'Connor stories I also own an old Bushnell Scope Chief 3x scope and the optics are fine on that. Whilst true that optical coatings have improved over the last 4 decades or so I am not convinced that they were the handicap some think.
In my little collection of scopes I have a few 4x ones from different makers. Due to the cost of good quality optics I have to buy used and currently the selection is:-
4z36 Pecar Champion .................... an excellent scope in every way!
4x32 Khales 4 L2 light alloy tubed with German Trip Post reticle. Excellent optically but lacks eye relief
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Zeiss Jena ZF4/N Dedicated scope for the Brno ZKK 601 with Tri Post again excellent
Hensoldt Duraltyn 4x Alloy light and very clear optics only fine cross hair though.
Carl Zeiss 4x32 optically superb good reticle but of rail mount construction.
Now as the thread is about a cartridge with authoritive recoil then not only strength of construction has to be considered but also of great importance is eye relief. I would not put that Khales of mine on my .458 for instance nor the 9.3x57. Simply not enough eye relief for comfort and safety.
Now although it feels strange to say this I do think that Swampman has a valid point in that we modern shooters seem to require much more magnification in our scopes than our fore bears. I am not convinced this is a good thing either (higher magnification that is). When I really I started stalking, deer hunting that is, it was with a 6x42 scope as that's what the "experts" recommended. I did use the 4x Leupold a bit but found the objective a bit on the small side as was the one on the Meopta Prepov 3x scope I acquired. Having very limited funds I was lucky to stumble upon a Pecar 3-7x36 scope for a good price and brought it. Then the Zeiis Jena for the ZKK 601 came along the the 6x42 on that rifle found a home on another and the Zeiss was fitted.
Quality of the optics became much more important for my hunting after an incident in poor weather where it was impossible to tell which way the Roe Doe was facing. The budget Japanese scope fitted that day to my .270 BSA was removed and replaced as soon as possible after that. The 270 ripped it apart after 60 rounds anyway under recoil
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Now except for the slightly smaller objective for best possible light transmission for the use that the OP wants I cannot see how something like the Leupold 2.5-8 vari X111 or what ever the new version is called of this would be a bad choice. It has eye relief enough and optically it's quite good with a range of magnification that would suit most needs. The big European scope makers do scopes especially for such heavy rifles that are meant of dangerous game these tend to be 1-4 magnification but a few also offer scopes in the 1.5-6x42 range I believe. Carl Zeiss do at least but boy do they charge for their products. Meopta offer one too at a more reasonable cost but still expensive.
