madman2 -

Here's your basic scenario with these highly confusing parameters:

If you disable (or leave the box unchecked) the item 'Don't delete files', all files that have been changed on the destination, i.e., they are different than the equivalent files on the source, will be deleted.

If you enable (or check the box) the item, the files which are different will be retained, using the same file name.

With regard to your interval question, if you set the internal to 4 hours, the task will run every 4 hours, no matter how long it takes. If you have a small amount of data, the task will run as long as it takes to complete, then will run again at the next 4 hour interval.

Make sense?

-Asaph