With "https://www.evga.com/products/product.aspx?pn=220-G3-0550-Y1"
You only have One EPS 8-Pin 4x4 and you Do need the Second 4-Pin.
You may want to rethink the PSU you are going with, a 750 Gold would have been best if using a AMD Ryzen 7 CPU
Where did you Hear "I hear that the 4 pin is optional anyways, so can I just plug in the 8 pin?" this?
Need More Hardware Info like Make and Model Numbers.
CPU
GPU
Memory
 
This Question might be best asked on the AUSU Support as well, at least they should know this being their MB that is.
 
 
From https://www.asus.com/us/Motherboards/ROG-CROSSHAIR-VI-HERO/specifications/
2 x RGB Header(s)
1 x USB 3.1 Gen 1 connector(s) support(s) additional 2 USB 3.1 Gen 1 port(s)
1 x USB 2.0 connector(s) support(s) additional 2 USB 2.0 port(s)
1 x M.2 Socket 3 with M key, type 2242/2260/2280/22110 storage devices support
1 x TPM header
8 x SATA 6Gb/s connector(s)
1 x CPU Fan connector(s)
1 x CPU OPT Fan connector(s)
3 x Chassis Fan connector(s)
1 x W_PUMP+ connector
1 x AIO_PUMP connector
1 x 8-pin EATX 12 V Power connector
1 x 4-pin EATX 12 V_1 Power connector(s)
1 x 24-pin EATX Power connector(s)
1 x Front panel audio connector(s) (AAFP)
1 x Slow Mode switch(es)
1 x Thermal sensor connector(s)
1 x Reset button(s)
1 x LN2 Mode jumper(s)
1 x ROG extension (ROG_EXT) header(s)
1 x Safe Boot button
1 x ReTry button
1 x System panel connector
2 x 3D Mount screw port(s)
1 x W_IN header
1 x W_OUT header
1 x W_FLOW header
1 x Start button
1 x USB 3.1 Gen 2 front panel connector

hbzk100
I confirmed it after typing my post above, the manual and an Asus Rep on overclockers.net said the 4 pin is optional and should not affect normal overclocking.
 
But it is a bummer that the G3 550W doesn't come with that extra cable either way, oh well.

OK, Best of Luck.
BTW I pulled my 4-pin EATX 12V Power connector from my AMD MB and it would Not Post with the RYZEN 7 1800X set to 4025MHz.

Then why Bother putting it on the ASUS AMD MB in the First Place?
Link to the Forum Post would be great, but the Forum does not look like ASUS's Forum Site?
http://www.tomshardware.com/answers/id-3470957/crosshair-hero-cpu-power.html
Alternatively, you can use just the 8-pin socket of the motherboard (if you are not overclocking), as the 4-pin socket is optional to be plugged in (alongside the 8-pin).
http://www.tomshardware.com/answers/id-3363686/crosshair-hero-power-connectors.html
Not a big deal in the long run, have fun with your new computer, Game On.