As for designing for retina do I.... set up the image canvas to be the true size to my display device, say iPad screen size (not resolution size) Dimension size. Then set my dpi to 72....? Is DPI 72 or... the resolution to whatever the retina display is (I.e 4k, 1080p etc)
DPI is completely irrelevant. What matters is the number of pixels.
A non-retina iPhone, for instance, is 320 pixels across. A retina iPhone is 640. (The iPhone 6's retina is 750 pixels wide).
What makes a device 'retina' is that it uses more than one physical pixel to create a virtual pixel. An non-retina iPhone and an iPhone 5 are both 320 virtual pixels wide. But the retina screen has more physical pixels, so you can create a sharper image.
So, if you need to design an icon that is 100x100pixels, you'd also want to create one at 200x200 pixels for retina screens. They'd both look the same size-wise on screen, but the latter will have more data, and therefore more detail.
Generally speaking retina screens are 144 ppi (72x2).
But it's not nearly as simple, here's a great guide that will tell you everything you need to know about retina and dpi in general.