iPad5 will be machined like the mini, less thick and less wide than the iPad4, but same size screen.

iPad3 got discontinued because it is the same as the iPad4, except iPad4 has a better CPU and lightning connector. It was not considered by Apple as a new annual model. In fact, Brazil has a class action law suit that iPad3 owners get the iPad4 for free or some sort of compensation because Apple did not adequately prepare the public of the refresh and were still selling 3's until they were quietly replaced by 4's within the same cycle year, or something to that effect.

iPad1 and 2 - normal resolution
iPad3 and 4 - retina resolution (once you start and get accustomed to retina, you will not like normal reso, it will appear faded and blurry)

If cost is an issue, iPad3's on the used market are a best buy.

And if you plan to jailbreak in able to mod your iPad or get apps, it is best to do it before Apple closes the hole with the next iOS update.

The next iPad mini will likely be retina screen, no jailbreak of course.