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.
Another article on Wikipedia about decimal separators rehashes and adds to some of the comma article. It goes as far as listing countries as dot countries, comma countries, and momayyez countries. I thought it was pretty amusing that the world was being divided up by the decimal separator they use and that the Middle East didn’t follow either convention but came up with something completely different that I’ve never even heard of. However, the article does at least mention that some publishing house manuals of style do not include a comma in numbers 1000 to 9999.
That brings up one of the flaws of Wikipedia: tons of information but not always an answer. And sometimes way more information and opinion than you really want or need.
Although the Wikipedia article about the momayyez says it is a forward slash, in the article it appears as a comma. Searching for images of a momayyez on Google results in pictures of a guy with a mustache and a woman in a cubicle. I played around with the unicode character in Word and came up with this image that shows that the momayyez looks a little different and does not descend like a comma. I will reiterate that since the momayyez is a decimal separator, the first image represents 40 to three decimal places and the next one 40,000.