In MainType 4, in the Sample area, how do I view a specimen that contains small caps?
That depends on the font. If the Small Capital glyphs have no character mappings and do not appear in the Character pane unless “Show all glyphs” (click the G icon) is enabled, then “Add to sample” will be greyed out on the right-click menu in the Character pane.
Otherwise, right-click on the Small Capitals characters in the Character pane, and add them to the Sample pane one by one. Then in Tools, Options, Text Samples save the new text sample for reuse. The Small Capital glyphs will show up as rectangles in the Options dialogue since the system font doesn’t support the Private Use Area, but the text sample will show the Small Capitals glyphs in the Sample pane.