If, in the source document, you insert a cross-reference to the paragraph in question, Word will automatically bookmark it selecting the cross-reference and pressing Dhift-F9 will reveal the bookmark name Word has assigned. The INCLUDETEXT field can then reference that bookmark.

However, if all you want to do is to reference the paragraph number in the source, a slightly different approach is required:
• In the source document, insert a cross-reference to the paragraph, using the 'paragraph number' option.
• Select the inserted cross-reference and press Shift-F9 the enclose it in a pair of field braces (e.g. { 4.2 })
• Edit the field, so as to turn it into a SET field, thus: {SET BkMk "4.2"}, where BkMk is a bookmark name, then press F9 to update the field (which will disappear).
• In the target document an INCLUDETEXT field that references the bookmark used in the SET field.

Note: since you won't be able to see the SET field, making it prone to accidental deletion, it might be best to create it in the document header or footer, where it is less likely to be deleted by accident.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]