I haven't done a complete finite-element analysis of the T4R body and frame, so I can't really say how sturdy it is, but people do offroad them and haul large trailers, and I haven't heard of any failure of structural integrity. Although there are many accounts of the V8s collapsing the frame at the engine mounts on the T4R and FJC. I trust Toyota has built these trucks well, and I think the body structure is carrying more of the forces and the frame has been lightened to take advantage of this construction technique.
I haven’t heard of any account of the V8 engine collapsing the frame of a 4Runner. And even if that were true, I’d say it is more of an engine mount issue and not the fault of the frame itself. The FJ Cruiser doesn’t have a V8 engine, so I’m not sure what you mean there. The only negative thing I’ve heard about the FJ Cruiser is that a number of owners who modify their rigs and run them offroad have had some issues with frame where the engine (a V6, by the way) mounts to it. I don’t know enough about the issue to go too far into it, but I will say that the 4Runner does not suffer from the same problem. Plenty of guys do hardcore offroading in modified 4Runners and you just don’t see people complaining about any issues like that.