Big two go out to sump smaller two are returns.
If the holes are same size look at bulkheads.They will accept different size pipe.
Did you purchase or get the "Mega Overflow " kit with it ?
Aqueon sells the 'Mega overflow separately and two kits should run you around $120..
You could DIY but it helps to know what you are building..
https://www.google.com/aclk?sa=L&ai...wio_-yTnc7aAhUOMt8KHWXRADQQ0Qx6BAgAECY&adurl=
Those go in the overflow and then you need to plumb to and from sump..
Got a sump ?