AD5933 analogs for 4-terminal measurements
http://iopscience.iop.org/1742-6596/407/1/012019/pdf/1742-6596_407_1_012019.pdf is a paper you may not be able to get without being in a subscribing university, but you can try.  They add an analog front end to do what you're trying to do.  I'm including ref info and the abstract.


  Interfacing the AD5933 for bio-impedance measurements with front ends providing galvanostatic or potentiostatic excitation, Uwe
  Pliquett, Andreas Barthel, Journal of Physics: Conference Series 407
  (2012)
  
  Institut für Bioprozess- und Analysenmeßtechnik, Heilbad
  Heiligenstadt, Germany Email: uwe.pliquett@iba-heiligenstadt.de
  Abstract The AD5933 1, a specialized single chip impedance analyzer,
  made by Analog Devices, is basically not intended for use with four
  electrode interface. Due to electrochemical phenomena at the
  electrodes connecting the material under test (MUT), especially in the
  low frequency region below 100 kHz, a two electrode interface
  generates considerable errors during the measurement. Thus, for most
  application in bio-impedance measurement only a four electrode
  interface can guarantee reliable results. Here we show how a four
  electrode interface with galvanostatic excitation but also for
  potentiostatic excitation can be realized by just a few external
  components.


A similar arrangement is shown at http://www.instructables.com/id/Body-Composition-using-BIA/?ALLSTEPS


From the above instructables link