Nominally, you can take the exam even if you are not in school, but it is very difficult to register. However, if you register, it should be CET-4, not CET-4. If it is training before going abroad, I recommend the German study preparation department in Tongji, which is very professional and powerful. You can go out after studying for a year and taking the exam again. The other one is the Goethe Institute, but that one focuses on oral English and is not suitable for exams.