Well, let's see...

Over .4 and only one gun for 90%...

My choice would probably be a 458 Win Mag because the guns are plentiful, ammo is cheap for that genre and if you are into reloading, bullets abound for any purpose. The 300 grain Partition would be a nice deer/hog bullet loaded down and the Woodleigh 400 gr semi-spitzer would do for deer up to big bears here in this hemisphere or plains game. The heavies are out there if you ever want or need them.
Oddly enough, now that I've made the case I feel the uncontrollable urge to own one

I'm assuming you mean a "general purpose" .40"+ rifle cartridge for all-around use so I'll pick my M77 .458 Win Mag. cuz it's very accurate and versatile for me as a hand-loader and the choice of .458" bullets is enormous.

The .458 Win is sort of the 30-06 of the large-bores cuz it's plentiful/common and will always get the job done in the hands of a skilled rifleman. Yes there are certainly more powerful rounds and some have greater range capabilities, etc, but all-in-all it's a great round that's not terribly abusive on the shooter. With mine I'd have confidence in using it out to 200 yards on deer/elk/black bear.

Well Bwana

If I absolutly positively have to do it I would be my .500 Jeffery loaded with my own 450 grn High BC expanding monometal bullet That bullet flatens out the mighty .500 cal. some 0-200 is no problem.

The Impala bullet 370 grn is not bad either.

Cheers,

Andr�

4 Bore Pistol, round ball and a handfull of Black.
Anything that's stupid enough to come within 20yds.