The Battlefield 4 beta is set to go live on October 1st at 04:00 PDT for early access users but you'll be able to pre-load the early access demo from this Sunday, September 29th, at 10:00 PDT (you can convert to your local time-zone here).
While the PC beta can be pre-loaded early it's unlikely to take too long but it never hurts to be prepared and means you can start playing immediately once DICE press the beta's 'on' switch.
According to Reddit those with "exclusive early access" can indeed start playing at 04:00 on October 1st. Pixelenemy reports that they have confirmed this.
Everyone else will have to wait until 04:00 on October 4th to start playing. DICE has announced that there will be no non-disclosure agreement over beta gameplay footage so expect a deluge of footage when the floodgates open for everyone.
You can check out details of what's actually included in the beta here.
Battlefield 4 is set to go on sale on October 28th in North America and November first in Europe for PC, PS3, and Xbox 360. The game will be released on PS4 and Xbox One later in November.