Depends on the publisher. IMO, 4500 is pretty much perfect for most magazines who set the limit at 5k. Anthologies will let you get away with more. I usually do 6-8k with them, but 10k is possible too. My recent acceptances (all anthologies) were: 5k, 7.9k, 3.7k, 3k, 8.7k, 10k, 8.8k. The only time I hear publishers explicitly say "this is the perfect zone," it's just around 4-5k. But they're all different.
@Tenderiser wrote a great FAQ on word counts over here that you may find useful. That said, @Seven Crowns is also right -- it depends on who you're submitting to, as different markets have different standards. My experience as a sci-fi short story writer says that the sweet spot is about 3-4000w. If your metric here is quality, rather than industry standard, then it's all subjective.
Hitting between 3k and 5k words, I believe, is ideal. I regularly read and submit to many magazines that publish short stories. If you write longer or shorter, there are markets for that, too, but I think most of the well-known (and not-so-known) magazines and journals tend to publish in the range I mention. Of course, there are exceptions, but when it comes to short stories, it's super hard to be the exception. It's all super hard. One of the first things I did when I was new to writing short stories was to train myself to write in this range. It meant paying closer attention to word counts than I usually did. But if you read a ton of stories that are in this range, which most of them will be, you can really learn what to do and what you can do in the space of under 5k words. The short fiction I've had published has all been around 4k words.
