Yonkyo? ehum... Yonkyo omote - 1. Ikkyo omote (whatever the attack might be). 2. You don't go all the way down, just untill ukes balance is broken forward and uke has one hand down. 3. Your hand on ukes elbow slides down and grip together with your other hand as if holding a sword. 4. Cut the 'sword' down, and pin ukes shoulder still holding the sword.
Yonkyo ura - 1. Ikkyo ura. 2-4 as yonkyo omote.
Yes.. yes.. there is much more to it than that, the indexfinger inside base nuckle use, the balance breaking, ashi sabaki, te sabaki etc. but hey, that's what training is for!