At work I edit a manual of design practices and from time to time people will write with suggested changes. Recently I had a guy who was asking me to add some information. In the e-mail he quoted a sentence from the manual and included the new information. He also edited some numbers I already had in the sentence to add a comma, making 3500 appear as 3,500.
I don’t know where I picked it up, but it seems like I learned at some point that you don’t need the comma in 4-digit numbers, but should include them with 5 digits and above. I looked through the manual and at least I was consistent. I did a search to see what the standard practice is and it was pretty useless. Wikipedia has a big long article about commas that says one standard is to use spaces (actually half spaces) between thousands, like this: 15 000 and another is to use commas like this: 15,000. Of course they didn’t use a 4-digit number. Naturally they had to add the crazy thing where commas are used as decimal points and periods are used for thousands in some countries (which I learned about when I got my very first HP calculator that could be set to display numbers either way). In a discussion on Wikipedia, one contributor flat out refuses to ever use commas in numbers because it is WRONG (his caps) regardless of what the Wikipedia style manual tells him to do. That’s insubordination.