How many GB are in my iPhone 4?

It doesn't say in the back. And in settings it says 28.5 gb in capacity? :/ is that a 32gb iPhone?

You have 32 gb, but it only says 28.5 because the system (IOS) takes up alot of space.

16-64 gb

Yes. You don't get all 32 since some is used to store iOS, so the 28.5GB is what's left.
Same for all other smartphones, computers as well.