Seller.... If the lowest your sister will go is $4k, you just set the price- $4k. As a person that hates the process of selling machines to strangers, I would suggest your ad look something like this- POTC- purchased brand new, original owner.- $4k firm. Willing to be that moves it out much sooner than later, especially with lack of detailed pics.
As someone that has owned 5 POTCs, I can tell, even from the blurry pics, that you have what appears to be a nice original. The more pictures you can provide, the faster it will move. Good luck!