Answer: 4m
Step-by-step explanation:
Since the angle of incidence of a plane mirror can be anything from 0 to 90°
Assuming that the place is a perfectly square 4×4m room
The incident ray would be 45° for the choir(object) at a 4m distance, this is still within the range of values.
We do not forget also, that the focal length of a plane mirror is infinity, the organist would in fact see farther than 4m if need be. And wider