Renaming the headings in the navigation pane without changing the "real headings" in the content

 

Assuming that you also want just "Part A" in your TOC, you can accomplish this with a style separator.

1. Create a style with formatting identical to your heading style but with outline level of Body Text.
2. Using the heading style, type "Part A" and press Enter.
3. Apply the non-heading style and type " [1234]."
4. Place the insertion point anywhere in the heading paragraph and press Alt+Ctrl+Enter to insert the style separator.
5. The two paragraphs will appear to join, but only the heading text will appear in the Navigation pane and TOC.