Rokkyo is apparently what we call hiji-kimeosae. Its a technique used against a punch where you enter outside the shoulder, twisting the arm outside and upwards and bringing Uke to the ground by the shoulder. You can easily twist a knife from Ukes hand after it's done.
Gokkyo is a variant on Ikkyo designed specifically to avoid a knife in Ukes hand. You come inside and grip the wrist in the opposite way and then always go outside to finish the technique. Better than cutting your wrist...
Not everybody can work Yonkyo, it's not a control you can demonstrate and see copied straight away. The only way to get it working is to practice it until you get it enough to recognise it. Most techniques that don't work are just done badly, although some people just need a little extra something added to the control to cause pain to start. Anyhow, nobody's immune to being brought to the ground by Yonkyo and in all honesty Uke will have one hell of a stuggle to get up and no chance of adoiding atemi.