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What I’ve earlier heard is that Hiroshima was warned a couple of weeks previously that it might be bombed — as were several surrounding cities. This without reference to the kind of bomb, as the Allies were considering conventional bombings of the area. Hiroshima wasn’t the originally intended target of its nuke — there being no GPS in 1945, the bomber had to see the target.

The fourth bomb didn’t exist yet, but its components were expected to be ready to assemble in a few weeks. Japan surrendered before it was needed.

Hiroshima was chosen because it was one of the few significant Japanese cities that hadn't been firebombed. We wanted them to see what the bomb would do, without prejudice from previous damage. As you note, it was not the only city on that list, just the one with clear skies that morning.

Here's a list of cities that were firebombed and the percentage of the area that was destroyed. It doesn't have casualty figures, but you can estimate those and it will be a very large number. US firebombing campaign, 1945

There's a map on the right that shows the parts of Tokyo, Osaka, Kobe, and Nagoja that were destroyed by the bombing. If you stayed in a Japanese city in the summer of 1945, you had a death wish. If we had continued the campaign and not used the nukes, they would have eventually surrendered, and we probably would have incinerated another 500,000 or 1,000,000 Japanese. But we would not have used nuclear weapons, and that's the liberal way. ;)