You don`t need to get the nerve to do yonkyo, it`s just a bonus if you do. If you`ve taken uke`s balance, and use the stretch, it should still work fine. There are four nerve points for yonkyo, two on each side of the arm, left and right. Don`t try to find them, the harder you try, the less effective they are. If you the knuckle at the base of the index finger softly, you`ll have a better chance.
Gokkyo is a variation on Ikkyo, used for knife attacks. Basically reverses the way you take uke`s arm to lessen your chances of getting cut. The finishing pin is a bit more brutal too, in order to get uke to release the knife, if he can
Rokkyo as has been taught to me here in the UK, is pretty much the same as the hiji shime taught to me in Japan.
I`ve not personally heard of nanakyo or higher.