FCS Express 4 density plots have 25 discrete color levels, whereas FCS Express 6 and 5 density plots are rendered with a finer color palette of 256 color levels. This results in fewer events being in the outermost, or "Outlier dot," density level. Thus, in most FCS Express 6 plots, you will see fewer dots than in V4 (e.g., Sample 1, below), or in some cases, none at all (e.g., Sample 2).
If you wish to see a similar number of outlier dots in your new V6 layout as you saw in V4, try changing the plot type to Contour. Rainbow and Flame Contour plots have 25 discrete color levels in both V4, V5, and V6.