Didn't preorder, was able to order online this morning on the iPad and select pickup in store, got the ready to pickup email within an hour.
Are we able to use the power cord from the ATV 3?
Yes, I just swapped the aTV 4 in place of a 3 in a bedroom and reused all the cables.
Have you found or tried Plex or Simplex?
SimpleX shows up in the App store for $2.99, Plex is not there yet. Not much of a Plex fan myself, I've always preferred XBMC/Kodi and Air Video. The Air Video app for the aTV4 is quite nice, been using it all afternoon, at the moment a $3.99 purchase.
Is Amazon Prime airplaying well? Look good? Sound good? surround sound?
Amazon Prime from an iPad Air 2 worked just fine as you'd expect. Didn't run the audio through an AVR just yet.
After setup, have any of your previously purchased apps showed up on the ATV?
Anything you purchased previously shows up under a "Purchased Apps" tab with a little install icon option.
A few quick notes, I setup the 4 w/ my iPhone over bluetooth following these instructions and it was very nice not having to manually enter and re-enter passwords from scratch:
make sure you are on iOS 9.1
searching with Siri is nice, everything you'd want it to be, at least within the limited apps allowed so far, I can see this evolving into a very appreciated feature once it's opened up more...though I think my favorite aspect of the new remote is the touch scroll left or right while within a video...very nice being able to see and then go to just where you want within a split second. I'll move it into our living room later and can test more audio related aspects...