Harry & Kyteflyer,
Thank you both ... looks like my iPad Mini 4 is a keeper.
I checked the charger from an old iPhone 4 and it is 5W. Likewise, the charger from my iPad Mini 1 is 5W.
However, the iPad Mini 4 came with a 12W wall charger.
Apparently the USB on my old desktop (Windows Vista 2007) is out of its league trying to charge the more power hungry Mini 4.
Sure do like my new Mini:
Retina display versus non-retina
128GB versus 16GB of storage
Not limited to iOS 9.3.5
Faster processor, some apps load noticeably quicker