When I make a category graph in Prism4, I want to change the gap after the last category. Also I don't want to label every category.

When you make a category graph, Prism 4 picks the beginning and end of the X axis and doesn't let you change its choice. But there is a work around:

1. Double click on the X axis to bring up Format Axes.
2. Note the popup for whether you want the X axis to plot row numbers or labels. Choose (for now) row numbers instead of labels
3. Uncheck Auto for the axis limits (which you could not uncheck before), and enter limits you want. Each category corresponds to a row on the data table, and you should use the row numbers to guide you to pick sensible limits.
4. Change back to row labels from row numbers.

You can also change the labelling interval within the Format Axis dialog. If you enter "10" then every tenth category will be labeled.

Prism 3 and earlier do not make category graphs directly. For a workaround, click here.