So I haven't seen episodes 4-6 yet, but tell me anyway. San now knows that Sucat(?) is evil right And that he's…
Yes he knows that sucat is evil. It hasn't shown that he has a plan or is actively trying to stop him and the dying guy other than having a fake stone and hiding the real one. If by "the woman" you mean vee, at the end of ep 6 a lady that looks like vad comes in and the preview shows them trying to figure out who the real vad is between vee and the vad look-alike.
how is it connected to the prequel?should i watch it before this?
it brings back the main lead and some of the side characters from the original so it might be good just to get the back story i don't know how important it is though since it hasn't been released online yet so i haven't been able to see the sequel
According to the director no, but KKH's character has SK's character saved in his phone as the bastard i loved (official translation). There are longing looks between the two of them, as well as brief talks of their past together. I would say it's heavily implied even though the director said it's not cannon. Later the director said that you can interpret it how ever you like, so I would leave it to your interpretation once you get the chance to watch it.
im confused... im four episodes in and i dont know how ki baek and jung heon know each other it showed they've know each other since they were students but were they friends did they just go to the same school i feel like i missed something