>are they redudant?
Yes. As an example, the 9.7" iPP is the same as the iPad Air.
>If my app looks good in all the device views and runs correctly on all the simulators, which actual devices can I be reasonably certain it will work on?
Unknown without real world confirmation, but most, on a good day, I think. Keep in mind the simulator is not an emulator and it's screen is your computer display.
Simulators are meant to evaluate your UI's logical flow more than anything.
That said, and since it is not reasonable for every dev to own each device, it becomes a leap of faith, and something that is learned over time whether or not one's layouts perform as hoped in the real world. The more app's you put in the store, the more you'll get a feel for the nuances of how your layouts etc. work on various screens.
App Review will have an opinion as well, but don't treat that process as a testing service. Speaking of which, don't ignore that TestFlight can bring more devices into play if you want to cover that base. And of course your users will let you know if they're not happy about how it looks as well.
>4S - not sure if this is just a typo
iPhone apps can be run in 1x/2x emulation mode on iPad...that scheme replicates a 4S screen.