You have the basis of it right.
If your first cab is 4ohm, load your second cab with 16ohm speakers and wire them parallel (16/4=4ohm). Then, chain the two cabs together in series (4x2=8ohm).
For an excellent beginners guide to series/parallel cab wiring, download the Dual Rectifier manual off the Mesa/Boogie website. Near the back is a section with some excellent pictures of wiring methods and how to run multiple cabs and what-not.