The difference between the two camera types.
With a tk2dCamera - it will attempt to make everything you see in the game window 1:1 all the time. So, if you have a 2048x1536 image, it will ALWAYS display at that size on any device. In your example, when running at iPad1/2 native res, you'll see only a quarter of the image. The origin of the tk2dCamera is the bottom left of the screen, so you'll see the bottom left quarter of your image. In order to get this to display to fill the screen here, you will need resolution overrides for 1024x768, and set the scale to 0.5 there. Thats telling Unity to display everything at 0.5x at 1024x768. Everything will then fit the display. On iPhone4, you'll see a cropped 960x640 image there, unless you use a resolution override.
With a normal ortho camera - set the ortho size of the camera to a number. This can be any number. Set the same number to the ortho size parameter in the sprite collection and commit. Also, make sure the sprite collection "target height" parameter has the correct resolution there, so if you're targetting 2048x1536, then the value there should be 1536. When you create this sprite in the viewport, it should fill the screen, regardless of the resolution. Even if your res = 1024x768, it should fill the screen. It will scale the image to fit the height on an iPhone4, but you'll see 2 bars on either side.
Hope that makes sense.
I'll post about different aspect ratios next - that is very game dependent. The more I know about your game and how you wish to use the sprites, the better recommendation I can make here.