Reason for Four Inputs (U+ U- I+ I-) on an impedance analyzer?

This is something I've wondered about for quite a while and even after combing through some HP and Agilent manuals, I still don't have the exact answer. On a lot of impedance analyzers and RLC meters, I'll see BNC connectors on the front panel that are marked "U+ U- I+ I-" or more commonly "Hcur Lcur Hpot Lpot." I'm sure this refers to differential voltage and current-sensing amplifiers inside the instrument but I had a few questions about that...

- Why are there four inputs and a ground (vs maybe just one or two inputs, plus ground)?

- Is there a name for this "architecture?"

- Has anyone come across a full schematic of this input "architecture?" (I have only found summaries)

Wiki (and Google) is your friend.

Hardy har har. A simple "it's called four-terminal sensing" will do. Thanks though.

Also, Google "Kelvin 4 wire".