RX 470 comparison

I see most of the users here saying that I should buy RX 470 8GB from sapphire for mining.
Is there such a big difference between 4GB and 8GB?

And why not some other brand like ASUS or MSI?

Pretty sure 4gb can only run one instance where 8gb can run two

That's not how it works. The 8GB version doesn't run two instances it runs a single, just like the 4GB does.

4GB can run 2 instances, but 8GB should be able to run 3 or 4.
when I try to run 3 instances on a rig with both 470 and 480, only the 480s mine.
when I run 2 instances, all mine.
anyone saying that they just run a single instance is wrong.

With ZEC mining there isn't much of a difference. The 4GBs are cheaper and have a lower ROI period. That's why I get the 4GBs and recommend other do the same.