I think it's very possible that they will sell out. However I am praying that they won't because I plan to buy the phone at least a few days after release if not a week or two, and if it is sold out completely this would be a serious problem for me. Also I don't think the phone selling faster will make samsung move any faster to release 4g.
Also just because the Captivate and Vibrant didn't sell out doesn't mean the Epic 4G won't. It is sprint's high end slider and is a 4g phone. The captivate has no chance competing with the iPhone, and the Vibrant is on a none too popular carrier and is gimped compared to the epic 4g. I do hope that the Epic 4G stays in stock and gets discounts like its cousins did though.