>>31
For futatsu, it's 二つ, not 四つ. I did some research on your question, but didn't find a clear answer. This post says that some consider ２つ incorrect. This post says that kanji is preferred in vertical writing. This post says that numbers are usually only written in kanji when the writer is going for an old fashioned look. I'm not sure what to take out of this, but hopefully this will help you find an answer.