I've only ever had male cats (Strange but true. Especially since all except one were found and not actually 'chosen'.) and none of them as ever sprayed. Spraying is really behavioral and as long as you neuter them before they sexully mature you should be fine. Most cats don't mature until about 5-6 mos. So I'd agree with the sometime between 4-6 month range. I've always done it closer to 6 mos. but if you're worried do it at 4. They can do it at a very young age. My pound kitty ws done at 6 wks.!But, as Ianthe said, it's because they HAVE to and, personally, I think that's a lot to put such a little fellow through. And, althouh studies say otherwise, and I understand the necessity, I'm not convinced that it's the healthiest way to go. But that's just my 2 cents.
And Mealie, as emma, said it's never too late to spay or neuter!