Generally you should create your art for the highest-resolution you're planning to support and then scale it down (there are tools out there that do this automatically).
That eliminates the resolution-problem and leaves you with the problem of the aspect-ratio. There's no definitive solution to this problem, you'll have to figure out what works best for your game (scaling/cropping, letterboxing etc.). Here's a similar question.
When your screen-sizes have significant differences in size (eg. a desktop-computer vs. a smartphone), then you'll have to adapt your design to these different sizes. Text that's perfectly readable on a big screen might become unreadable on a mobile-device. On the other hand a HUD/GUI of a mobile-screen applied to a large screen might look clunky and not appropriate for the device.
Please note that I'm talking about physical-size and not resolution here. I'm aware that a iPad 4 has a higher resolution than most desktop-computer monitors, but these pixels are so dense, that you still design your layout for 1024x768.