How to remove the page numbers on the 4 beginning pages, and start to number the pages starting the fifth page?

Put the cursor at the end of page 4. Click on 'Page Layout', 'Breaks', and select 'Continuous'. You have inserted a section break there. The whole document has been split into two sections in terms of header and footer.

Double-click on the page number '5', and select '5'. You will see that the tab 'Design' has automatically been selected, and the 'Link to Previous' button is down. That means that the header and footer in the current section contain the same content as in the previous section.

Click on 'Link to Previous' button, to let it be up. You have un-linked the current section from the previous section, so that the header and footer in the current section do NOT contain the same content as in the previous section.

Double-click on the page number '4', select number '4', and delete the number '4'. You will see that the pages numbers from 1 to 3 have automatically deleted, but page numbers starting 5 still remain.

Double-click on the page number '5', and select '5' again. Click on 'Page Number', 'Format Page Numbers...'. Choose 'Start at 1'. Click on 'OK'. You will see that the page number on the fifth page has become '1', and the page number on the sixth page '2', and so on and so forth.