Well Venga if you are able to pick up all 4 of them in one shot for $500 a piece, I would jump on that offer. The DF's at that price are a very good buy as they tend to sell at 3 times book. At $500 for the regular issue it's a little high but not much as they will aquire a lot of value over the years.
Don't forget that you should take 12 off that total of the remaining notes for the regular sets and 9 for the DF's.
Personally I collect then for their rarety and their history and yes they are all AU's but remember how much handling they have had over the years, So you might see some glue spots and other minor things but they should all be about the same. I love them and they are the prize of my collection and in 15 more years my retirement. I can't get the kids to get in to collecting banknotes so it will go back to other collectors.
So Venga to answqer your question you should pull the trigger and jump on those for that price you're getting a deal. If you don't want them send me a PM and I'll buy them.
Denis