Can I ask a question, what do you call perfect for one handed operation with the buttons?
The Oasis screen size is going to be the same physical screen size as your 6" Kindle 4. Both are 6". The Oasis does have physical page turn buttons which are great but it doesn't have a select or cursor pad in hardware buttons, so that might be something you would be lacking. No newer Kindle has these buttons.
In terms of holding a Kindle or eReader in one hand and using it to page turn and grip the device, the Kindle Oasis is definitely far better than previous Kindles for me. Here's a good image of it in action.
http://ebookfriendly.com/wp-content/...ts-540x438.jpg
At a pinch you can use with your thumb to highlight text etc on the screen, if that's the way you're doing it with your Kindle 4 then there should be no difference whatsoever. If you're using that keypad to bring up the keyboard, the the 5 way controller, press menu or home or back then that won't be anything you'll be able to do on a newer Kindle unfortunately. What you will have though is hardware page turn buttons that operate in either hand left or right by rotating the device.
Definitely there should be a market for smaller eReaders. However, I think the reason why there isn't is because most people use their mobile phones for it instead. The problem is most eReader manufacturers have found what they call the sweet spot at 6" and likewise a lot of mobile phone manufacturers have similar found the sweet spot at around 5-5.5".