For people unfamiliar with "gauge": 4(12) gauge basically means that your barrel diameter is equal to the diameter of a lead ball weighing 1/4(1/12) pound.

In case of this 4 gauge shotgun, we're talking about a barrel diameter of almost 27mm.

That's larger than a WW I anti tank rifle bore, right? How did he not dislocate his shoulder?

He did a very good job keeping control of the gun.

Yes, that's a larger diameter than an anti tank rifle, which usually was between 12.7mm and 20mm. But compared to, say, a 20mm Lahti L39, this most likely uses a lot less propellant in the cartridge. A lot. Thus less muzzle energy, less recoil, less injuries.

For comparison, Rock Island had a four gauge and a .950 JDJ (that's ~24mm) in one of their auctions, and they took them out for some shooting. Here's the clip: link. And yes, that's Ian from FW in the background.