At a gala party, Xiao Li performed a Yue opera on stage and won praises. Lao Zhang said: "Xiao Li's Yue opera sings so well, it must be a southerner. ". Which of the following does not mean in Zhang's words? ['Not a Southerner, Yue opera cannot sing so well.', 'Only when the Yue opera sings well is the southerner.', 'As long as the Yue opera sings like Xiao Li, it is a southerner.', 'Unless Xiao Li is a Southerner, her Yue opera cannot sing so well.']