When adding page numbers on certain pages in a Word file, you need to insert page breaks to separate the page from other pages. Please see the steps below to insert the page number on page 4 as an example.
Part I: Insert Numbers to all pages
Step 1: Click the "Insert" tab from ribbon;
Step 2: Click "Page Numbers" in the "Header&Footer" area;
Step 3: Select the page number location and the format.
Part II: To Separate Page 4 from other Pages
Step 1: Move the cursor to the end of page 4;
Step 2: Click the "Breaks" command from the "Layout" tab, and click "Next Page" from the drop-down list;
Step 3: Double click the footer areas on page 5 to select the page number, and click "Link to Previous" from the "Design" tab in the ribbon;
Step 4: Move the cursor to the end of page 3, repeat step 2 to insert a section break;
Step 5: Double click the footer area on page 4 to select the page number, and click "Link to Previous" from the "Design" tab in the ribbon;
Step 6: Page 4 is now disconnected from other pages.
Part III: Format Page Number
Step 1: Double click the footer on page number 4 to select "4";
Step 2: Right-click and select "Format Page Numbers" from the dialog box;
Step 3: Type any number in the box "Start at" as the start page number;
Step 4: If you have more pages (or sections) to add page numbers, please repeat the steps above to disconnect from other pages and add page numbers on each page (or section);
Step 5: If you do not want page numbers on other pages, double click the footer and delete page numbers from other pages.