Math = working out how many mc's req for continous slide

Hi guys, math related question here…

I’ve got a horizontal image slider which displays 4 thumbnail images and any given time. And i would like it to always show 4 thumbnails even if there are less than 4 actual images available. So essentially i want it to repeat images in sequence. I know for an “infinite slider” as showing in this tutorial (http://www.kirupa.com/developer/mx/infinite.htm) that i need double the viewable thumbnails (i.e. if im showing 4, i need 8 in the mc). Anyway your help greatly appreciated!