I was always taught that from 1 - 10 I write the word (one, two, three, etc...) and everything else put the number (11, 12, 13, etc...)
My tutor corrects some and doesn't on others.
What is proper or, what were you taught?
With numbers, I always spell them out, no matter what the number is. 617 is six hundred and seventeen. 1,275 is one thousand, two hundred and seventy-five. 4 is four.
I make exceptions to my own rules, however. I don't write out addresses, phone numbers, decimals, or anything too big (like over a million).
Hope this helps!