Don't get hung up on the numbers, just be sure to follow the correct order. Sediment > Carbon>Membrane> DI.
The RO membrane goes after the carbon and before the DI, regardless of how many of each filter you have. I doubt you need this, but some use 2 sediment filters, then 2 carbon, then the membrane, and then DI. In this example, your membrane would be the 5th stage because the water goes through 4 stages prior to the RO (2 sed. and 2 carbon).
And while the clear housings can usually hold any of the other filter media, the white RO housing only holds an RO membrane.
Regarding moving things around, all you need to move is the tubing... The individual filters could theoretically be placed in any order as long as the water flows through sediment, then carbon, then RO, then DI.
I had to learn about this too and it was a lot easier to research it online than have someone try to explain it here... BRS, Buckeye Hydro, and many other websites (probably even wiki) can help you understand the differences. Fwiw, most just use a mixed bed.
Exactly. Since the first DI will exhaust fastest, if the second still seems useable then move it to the first position and let it remove more before you throw it out.
Color changing resin is a good way to determine when the di is exhausted.