Only 2 of many testimonials are showing

Basically, when you use the "Prevent Duplicates" option of Testimonials Premium, it limits each testimonials widget instance to those testimonials that are remaining of the testimonials pool. Since you only have 12 testimonials, your first testimonials widget is probably grabbing 10 and then leaving only 2 for the second.

I suggest setting the global settings (WP Admin > Testimonials > Settings >Selection tab > Limit to 5) and then making sure your WP Admin > Appearance > Widgets > Testimonials Widget also read limit 5.

Alternately, if you want to have 4 stacked testimonials widget rotations, then the limit should be 3. Why? 12 testimonials total, spread over 4 widget instances, means each widget should only pull 3 testimonials.

That should do the trick after you check "Clear Cache" on the Settings Premium tab and click Save.