I suggest 4GB too.
8GB usually just +10% performance but +20% higher price. if you can buy 8GB in 4GB price worth it, if no not.
(longern term you can sell 8GB cards at higher place)
but this is just my opinion
From my experience it doesn't matter as long as they have samsung or micron memory.
Samsung is easy to flash and can mine approx 8GB speed. (28-29 Mh ETH) You right
My 4GB without flashing just hashing (22 Mh)
with flash , -10 voltage approx 27-28Mh (i can get 28-29 but, eat more power. i reduced it)