Since you don't study for exams, you must improve your oral English and listening. Cambridge International English textbooks are authoritative and are used by many schools. In fact, business English can't be compared with CET-4 and CET-6. Business English is more formal and rigorous, that is, we use formal words instead of the words we use every day. When you have time, you can go to the bookstore to read some books such as business English. What others recommend may not be suitable for you, personal opinion. By the way, I used to take a class in an English training class in Haidian District. The training course is called It' s next to double Shopping Mall, Renmin University Station of Metro Line 4. Good luck!