Well considering XP supported 4gb, I don't think they would go backwards. Vista 32 bit supports 4gb of ram. The 64bit version support 16gb in Home Premium and 128gb's in 64bit Ultimate.
The final release was released to manufacturers in Early December. As far as buying it in the stores, I believed the retail version for the stores will be released at the end of the month or early February. That is last I heard.