Student or Learner
"I have been studying English for 4 years."
I have been studying English for four years.
I came across a video on YouTube where it says that it is incorrect to use a single number- 4, 7, 9. It says that we should write four, seven, nine, for example. It is difficult to me to mention the exact sentences. However, I am sure that the gist here is not to use a single number.
Currently, I am taking some notes from different booklets and I have seen a sentence where a single number being used. So, which one should I write in any English exam, four years or 4 years, please?
PS: I can't supply you that video because I have no good connection this moment.