In the first 2 stages the rules write that max 4 cores to be enabled only (+HT) , so can i use a 6 ,8... cpu with disabled cores since there isn't a socket limitation??

Yeah, i also don't know if Hyperthreading is allowed or not. Basicly the CPU has 4 physical cores but 8 "virtual aka split" ones.

What the rule table says - 4 cores max, native cpu cores, if with ht or without doesn´t matter. It is not allowed to use cpus with more cores and disable cores

So then the i7 7740x is okay, even though it's on a hedt platform since it only has 4 physical cores?

Correct.

On all stages that do not rule out HEDT explicitly you can use the 7740K