In your instance, did you try and set the banner count at "3" instead of "4", just to test in that manner?  And are you certain the fifth banner was configured correctly (I've never encountered a problem like the one you're describing...)

In the vendor system, I don't believe their is a way to have the banners rotate in terms of a "random" pattern.  They simply cycle through the order of how they were entered.  Even on the dnn site if you watch something like the sponsors module (refresh enough times) you will see that while all the sponsor logos rotate evenly, they still display in the same order time and again.  Perhaps there is a commercial module out there which would fix this (JanetSystems?)