What is a 4-wire voltage measurement?
From page 3-5 (thanks)
"4wV Allows operation with calibrators which provide a remote 4 wire sensing
capability by providing connections between INPUT HI and SENSE HI and
between INPUT LO and SENSE LO.
The 4wV annunciator in the left-hand display indicates that 4-wire sensing
is active."

I'm going to guess it's similar as remote sensing on a DC power supply.  When you want to exclude the voltage drop along the wires.  So very similar to 4-wire resistance.