Timed exclusive DLC was common place last generation and Bethesda famously released DLC for both Fallout 3 and Skyrim first on Xbox 360. It seems that any deals or technical issues have been worked out this time around however.
A fan took to Twitter to ask Bethesda's vice president of PR and marketing Pete Hines whether there would be any exclusive DLC. The reply was short and to the point: "we aren't doing a DLC exclusive with anyone."
Fallout 4 is due out on November 10 on PC, PS4 and Xbox One. Recently a season pass was confirmed, and it'll include all future content starting from early next year.