A: The age of four is a time when children are working out their ideas about men and women in more complex ways than previously, and many children show special sensitivities to issues of male and female at this time. Quirks such as your daughter's refusal to have a man teacher in tumbling class are common among four year olds. I would not make a huge issue of her continuing this particular class if she is putting up resistance. You might let it ride and see if things are different in three or four months.
I must add that it is always possible, although not very likely, that there is something amiss with this teacher. Without panicking, you could perhaps ask other mothers if any other children seem to have a negative reaction to this particular class. Children who are abused or molested may refuse to go back to activities where they were involved with the perpetrator without indicating in a clear way why they are refusing. Again, this is not the likely explanation and I would certainly try to be careful that you don't actually suggest this interpretation to your daughter. You might ask her in a calm and neutral way if the teacher ever did anything that made her upset or feel funny. If you don't feel comfortable with this issue or with her answer, consult your pediatrician.