The following workflow should be able to pad the document as described:
The route component will determine if the document is a multiple of 4 pages
Documents that already are a multiple of 4 pages are sent to the printer (or to some other operation such as imposition) without modification.
Other documents (with a number of pages which is not a multiple of 4) are then padded with a PDF file so the number of pages is a multiple of 4
The last page of the document (aka page -1) is then removed.
Lastly, a blank page is added to the end of the document.
The blank page size is based on the size of the last page of the document. This will allow the workflow to work with documents of any size