while it is true that the days of running more than one card in tandem are pretty much over, it is entirely up to you if you still want to do it or not.
if you already have both cards, and try running crossfire and don't get results you are satisfied with, and then decide to switch back to a single card, then yes, the 8GB card is the one you should keep.
If you have yet to purchase the 8GB card, then it is highly recommended you don't waste such money, since crossfire would probably not result in anything that is worth the investment, and going from 4GB to 8GB on the same GPU would not do much of anything to boost performance in games.
If you already own the 8GB card and were looking to get your hands on the 4GB for crossfire, don't.
it becomes the same argument made for buying the 8GB. waste of money.
perhaps taking the money you were going to spend on the second GPU (assuming you were going to buy it) and putting it towards a more powerful GPU instead as an upgrade then selling off your current GPU to recoup some of the cost for the upgrade would be a better idea.