4 year old says teacher bullied her at preschool- what should be done?

My four year old is in her second year at a preschool/elementary school. Last year the year started well but toward the end she seemed to be struggling with something and refused to talk about anthing re: her few mornings a week at school. I has umed that the problem lay with a couple of the girls not including her in play and so I spoke a few times with the her teachers about it. Each time they said there were no problems at all. About halfway thru the year one of the teachers ignored me completely when I picked up and dropped off my daughter and continued this strangely cold beavior until the end of the year. She was cool and distant to me on a daily basis but at each conference, alongside her co-teacher and boss, said only positives and niceties re: my daughter so I shrugged off her coolness as me and her just not hitting it off personally (never shrug off such a glaring problem). It nagged me that hse was so petty and unprofessional but I didn't piece it together. Despite all the positive feedback from her teachers and they're inistence my concerns were unfounded, my then 3 yo daughter seemed uncharacteristically mum about school and her once bubbly, highly spirited and sparkling personality seemed to be fading. I was worried but others suggested that it could be a phase etc. Well, my now 4 yo daughter is back at the school, now in preK (we debated on whether to resend her to this same school) and she suddenly "confessed" to me today that one of her teachers last year was "mean" to her and had "always yelled" at her "even at recess, lunch and circle time." This was not a casual conversation. Clearly she felt this was a scary subject to broach and was seeking help. I had asked her why she doesn't say hello or goodbye to her current teachers earlier in the day and I think it stirred up her wound. Later in her bath she asked to role play with a number of toys - yes, on her own she asked if one of the toys could be her teacher from last year and one her old teacher's boss. She told the boss that the old teacher had been mean but didn't yell at any of the other kids like she did to her and it had hurt her feelings. This role playing went on for a while. I told her I would speak to her old teacher and the boss and she responded by saying she'd like to talk to her too. She said she was "dumb" when she was 3 but now she is "smart" and knows that "not just hitting is bad" but so is "being mean." I cannot stress enough that this was not play acting and not a light-hearted fun convo. She was clearly getting this off her chest and seemed relieved about it all. I KNEW that something was up last year but didn't dig deep enough and now feel I made a huge mistake. My once incredibly bubbly and spirited child acts withdrawn, sullen, and nervous when in school. At home she is pretty much her true self but tellingly lost the confidence she had regained over the summer break only weeks after her return to school. What to do??? Do I call for a meeting and have her tell the teacher her feelings as she has requested? or do I go straight to the Director and tell it all? She says her new teachers are not mean and don't yell like the old one but this revelation has left me wondering about the place. She asked if her current teachers could keep her "safe" from the old teacher. I am not sure how to proceed. Your thoughts and advice truly appreciated. I feel over my head and treading carefully as I know the next steps could affect her views on school and her trust and faith in me as well. Thank you.