Shihonage (four corner throw) is actually a fairly dangerous technique. It's probably the technique that has injured more aikido students than any other. There are actually reported deaths from it in Japan. Apparently a couple of times, younger students at University got into it so much that they were throwing uke so hard that they suffered head injuries and broken necks. A few people died. I've known several people here who injured their shoulders pretty bad.
I was showing some of my BJJ classmates a version of shihonage koshinage that I like, and one of them asked about resistance, I simply said, okay, let's look at that, and then I showed them every place I could simply break the arm if someone really wanted to fight it, the point being that, once I get the arm extended into shihonage, you've already lost at that point, kind of like when someone locks in a kimura, and you probably want to fall as gently as possible out of the shihonage if you want to continue to fight. Now, resisting getting into shihonage is an entirely different manner, and one of the reasons we practice henka waza a fair amount, for myself, I'd probably do an udekimenage, or simply reverse into a sankyo or other movement. LOL....as my Shidoin says......"Always another way".....before he slams my butt.