The actress's character Elena Gilbert was presumed dead at the end of season three when she sacrificed her own life to save Matt Donovan (Zach Roerig).
However, Dobrev will now have to transform her human character into a vampire version without confusing it with her existing vampire role as Katherine Pierce.
She told Us Weekly: "I'm vampire-squared now! I'm playing two vampires, so my challenge is going to be making them different, as I have thus far.
"Before it was because one's a vampire, one's a human - it was easy - and now I have to try and make Elena a vampire."
Dobrev continued: "I know that Elena's going to struggle a lot with keeping her humanity. Up until now, that's what she's been struggling to hold on to. Her life is so supernatural, and now, without having a choice, she's going to have to live with it for the rest of her life."
The show will return with new episodes this autumn on The CW, and Dobrev is also looking forward to exploring the love triangle between Elena, Stefan Salvatore (Paul Wesley) and Damon Salvatore (Ian Somerhalder).
She added: "Finding out that she chose Stefan even though she actually met Damon first. . . there's so much going on and there are so many emotions. It's going to be a new beast going into Season 4."