If you only have a couple pages, I would suggest using any PDF editor software such as Adobe Acrobat. If you do not have a license, there are open source and free alternatives out there to combine and organize your pages. PDF Forge is one I used when I did not have the proper license level of Adobe.
If you are going to be creating frequent PDFs with multiple MXDs with more than a couple pages, you will need to create a script. That is going to be more time-consuming. As long as you only need 4 total pages the fastest way will be what I mentioned above.