Of course, these are just the vocabulary required by the exam, but many schools regard Band 4 as one of the graduation requirements, so I think a vocabulary of 4,200 should be mastered. If you are an English major, you need to master more vocabulary. I have a friend who is an English major. He had mastered eight or nine thousand words, and then he asked the teacher if his vocabulary was enough. His teacher said a lot to the effect that it was barely enough, but it would be better to master more words.
If you want to go abroad to take the IELTS TOEFL,
The vocabulary required by TOEFL IELTS is 12000.
The vocabulary requirement of gre is 15 100.