If you have it, use it. The cpu may or may not demand that power at any given time, but you will have power issues if the cpu demands aren't met.
Also, many boards for those cpus require use of both or simply don't work. Only the 3rd connector (4 or 8 pin) is optional for OC purposes.
Pin count on psu side is moot. The only concern is the motherboard side of the wire. Psu could be a 4,6,8,10 or 12 pin connection, makes no difference, it's just an attachment connector for the modular cable. The motherboard side for EPS (cpu) will be the 4+4 to match up with 4pin, 8pin, 12pin (4+4, 4+) or 16pin (4+4, 4+4) as needed.