EA noted in the announcement that beta players will be able to "learn maps, unlock items and build stats" for use in the October 29 release of Battlefield 4. Players can also carry over stats, unlocks and awards from the 360 and PS3 to the Xbox One or PS4 versions once they launch in November. Those looking to upgrade should check into the programs that offer deals towards buying the next-gen version of games they already own.
Those that sign up for Battlefield 4 Premium, pre-order Battlefield 4 Digital Deluxe or that have purchased Medal of Honor: Warfighter's deluxe or limited editions will be granted access to the "Exclusive" Battlefield 4 beta. The Beta's FAQ lists that the Exclusive Beta will also begin in early October - meanwhile this EA help article says that the Open Beta comes after the Exclusive one. We've reached out to EA for more information.
Update: The text of this article has been adjusted for clarity, and to reflect that further information may be forthcoming.