How long does Roblox UGC pending review usually take?

Four days isn't unheard of, especially for a first upload. Moderation on UGC is slower than it is for plain images, and it can sit there longer when there's a backlog. I wouldn't resubmit yet. You pay the upload fee again and you end up with two of the same item in the queue. Give it a week or so and only then look at whether something went wrong.