This is a very interesting question. There shouldn't be any reason in the world you'd need a magnum primer for a stick powder like a 4895 (nor Varget).

I think Eric is probably right re: 2 different approaches. As I understand it, the effect of opening up flash holes is to reduce the velocity of the hot priming mix particles whilst allowing a larger surface area of the powder to be exposed to them. A magnum primer may or may not do a similar thing. The difference between a regular and magnum primer can be that there's simply more of the standard priming mix in the cup. However, another way to make a magnum primer is to use a different mix/compound. See this rather wonderful article written by Allan Jones, formerly of CCI/Speer/ATK.

So, to me, if there's something to be gained by increasing the exposure of a 4895 or Varget to additional ignited priming compound, I'd personally choose flash hole alteration rather than going the magnum primer route, if for no other reason than the fact that you can't be certain what you're getting if/when using a magnum primer. And, even if you do find out for one certain brand at any given time, it'll change on you later. I'm still a bit skeptical that those powders need additional primer exposure in those small cases, though. Looking forward to what you learn!