hello
8 pin will be enough...;)
the xtra 4 pin can be used for really extreme OC (like Ln2), in order to improve stability/xtra support at a very high OC, among other things..
But for everyday use, and normal OC, the 8 pin alone is OK, Haswell isnt that voltage hungry
Its nothing wrong with having both of them plugged in tho..
Just make sure the 4 pin end up in the right direction (connector), or the PC will not start up, if you go for the dual connector setup that is...
My advice> skip the 4 pin