Ok, so I think it works like this:

You paid $4500. In order to reach the safety net you have to pay $1110.60. So you take that off the $4500 which leaves $3389.40 - that is the amount that will get the 80% back on.

Now that you have reached the limit for this this year, should you do another cycle that costs $4500, you will be able to claim back the 80% on the whole amount, the $4500.

When the new medicare year starts again (01/01/10) you have to reach the threashold again.

Hope that all makes sense