I do not believe we should characterize any preschool-aged child as engaging in bullying--which implies a degree of self-awareness about the power one holds and one's ability to use it to dominate another (who holds less) not just once or occasionally but in a systematic, repeated pattern. I think we are way too quick to pin that label even on older children and adolescents. Even where it applies, research shows there is a large overlap between children who feel they have been victimized by "bullies" and children who acknowledge they have victimized or "bullied" others.
In our early childhood settings, let's not rush toward the labeling and instead deal with the specifics of each situation that arises--and of each person. Many children have a lot to learn about becoming socially competent, just as they have a lot to learn about literacy, numeracy, and science. We don't punish or label them due to their deficits in literacy. And we should avoid punishing or labeling them due to not having been offered good models and good instruction in socially competent interactions. A good way to start is with the Pyramid of Social Competence as laid out (in numerous publications) by Lise Fox and her co-authors.