umm what are stages 4 5 and 6? mine is a 5 stage by kent marine. The Kent unit I have goes something like this:
When your feed water goes into the unit, it goes through the sediment filter first. Then it goes through the carbon block filter. After that, it goes through the membrane (the housing on top of the unit). Lastly, it goes through the vertical DI; (The last filter. It looks like it has tiny beads inside. The blue tubing is coming out of it..)

i think i paid 30-40 bucks for all my filters...sediment, carbon and di from airwaterice.com
the main membrane is more expensive though.