Ethereum will one day nid 8GB. Since it is not even possible to mine ETH with a 2GB gpu as of last Nov. At this rate, I tink 4GB limit will be arriving pretty soon. On top of that, 8GB gpu are usually quicker than 4GB since they are clocked higher. Also, u will be future proofing ur machine if u get the 8gb gpu.
I was still mining ETH on some 2GB cards into late Febuary or early March.
On the other hand, the "exceed 4GB limit" will probably never happen on ETH (it should eventually on ETC) as ETH is supposed to be going Proof of Stake long before the DAG file gets that big.
For gaming, 4GB works just fine on my 1440p monitor running Ashes. More and faster RAM (8GB cards seem to always have 8Ghs ram, 4GB usually has 7 and sometimes 6.6) wouldn't hurt for a future-proofing gamer though.
And no, most 4GB memory will NOT clock to 8Ghs - a lot if it does very good to get to 7.5