The CW drama was officially renewed earlier this month and will return with new episodes in late 2012.
"I have absolutely no idea [what's going to happen]," Accola told TV Guide. "Last year, I knew [season three] was going to be the year of the Originals, but this year I [know] absolutely nothing."
The actress, who plays Caroline, added: "But what I am personally excited to see is, what everyone's breaking point is going to be. Because at this point, everyone has really had to keep it together - what is gonna be the straw that starts to break the camel's back?"
Vampire Diaries star Nina Dobrev - who plays both new vampire Elena and her double Katherine - recently admitted that her changed role will make filming the fourth season a "challenge".
"I'm playing two vampires [now], so my challenge is going to be making them different," she said. "Before it was... one's a vampire, one's a human - it was easy - and now I have to try and make Elena a vampire."
The Vampire Diaries will return to Thursdays at 8/7c on The CW this fall. In the UK, the series airs on ITV2.
> The Vampire Diaries: What we want to see in season four