The answer to the question about how photos taken at mode 4k x 3k were reduced in quality to 800 x 600 received the following answer:
"When the battery is fully charged there's no indicator. The battery level indicator will appear on the Top left corner of the screen showing a battery half full, this means its low and you need to prepare to charge it. Once the battery runs out of charge you will get a message on the screen "Battery Exhausted" and you won't be able to take more pictures."
I understand the battery level indicator and that I cannot take pictures when the battery is dead; however, I don't see the relevance to my question. Could you please re-read my question below and answer?
"Took about 400 photos with my S8100 on a trip with image mode set to 4,000x3,000, however, after downloading them to my computer, many of the pictures taken in the middle of trip are only 800x600 quality. The sequence of about 50 of the 800x600 pictures is interrupted by 4,000x3,000 pictures many times, so I know the issue wasn't that the setting was accidentally changed. I chose to delete the pictures from the card when I downloaded, so I'm at a loss as to how this happened and what to do to prevent it in the future. We drove from the West coast to Mount Rushmore and unfortunately many of our best shots look like they were taken with an old camera phone. Any ideas on what happened and how to prevent in the future would be much appreciated."