Grammar, Usage / When to use numerals and when to spell out words? [2]

Sarah:

I posted this comment in the "contact us" section because it was not related to this thread (and I lost the original reference). "Contact Us" asked me to repost here:

Sarah:
Thank you for responding to my question about building an academic critique.

While perusing some of the other advice on the site I found a very thorough critique you did on someone's paper. I have a question about when to use numerals and when to spell out words. I learned that numbers greater than 10 and time (including years) use numerals.

In your advice you recommended the author change "4 years" to "four years". This is contrary to what I've learned. I would normally agree with the original author unless I was starting a sentence such as "Four years is a long time..." I would write "I went to school for 4 years." (I use The Gregg Reference Manual, 10 ed., paragraph 436, "Periods of Time". Am I misinterpreting it?)

Is there updated guidence or references that I should be following about use of numerals. (I use them A LOT!)

Thanks for helping create a great resource.
Dave