

Reasons:
Chasing decent calibre and lower noise. The 45Auto has only 27 grains powder capacity, versus 33 for 44 special, 39 for 44 rem and 42 for 45 colt. Will handle lower powder loads than the big cases + better load density, ignition and potential accuracy with tiny loads of fast burners in the case ( AS-30N/350, Bullseye, WST, 700X, red dot). 3-5 grain loads in the bigger cases can move around. Trailboss is always an option but being able to choose another 7-8 options adds versatility. Also getting a custom barrel does away with the bore size variability of lever guns.
The .45 has a couple of the best expanding bullets at low velocity, even by pistol standards, .70 cal mushrooms under 800fps impact. Thats a sub 5 grain load. They kill pigs provided its not too far out and accepting its more like archery kill times. can also step up to 1050fps with tougher bullets like the XTP etc.
I looked at 300AAC but as others have outlined they still haven't fine tuned great expanding subsonic bullets for it yet, ones available on our shores anyway. The 450 bushmaster was a good option but its a larger case, needs about twice the powder of 45 auto.
The No.4 is rated for 50kpsi so supersonic hunting can step 185 grain bullets out past 1700fps for a decent farm thumper- like a light loaded 44 lever.
The configurations or general appearance most go for are:
1. Delisle carbine, which was the ww2 covert supressed gun actually in 45ACP. You can still get these done, obviously the supressor is a fake, so Im not overly interested.

2.L42A1 sniper rifle (the ones in pic are 223 btw), I quite like the general stock configs.

Also there are various synthetic and various sporter stocked versions. I'd consider an aftermarket stock but dont know much about options for a No.4? Any blokes who do feel free to chime in..