why two pair of terminals are used in a shunt?
Imagine the shunt looks something like this, where R1~R4 represent connection and wire resistances, and are variable with tightness of the connections, temperature and so on. R5 is the shunt resistance itself (usually made of a low temperature coefficient alloy such as Manganin or Constantan. 







simulate this circuit &ndash; Schematic created using CircuitLab

Provided only that the meter is high resistance compared to the values of the resistors the voltage measured will accurately reflect the current I1 multiplied by R5 (only).  

It is important to use the larger connections for the high current and the smaller ones for the measured voltage because the larger ones are (or should be) on the 'outside' of the smaller ones. If you were to just parallel them all in my example there would be a large (and variable) 5m\$\Omega\$ resistance on each side so the error would be a very large (and variable) 10% of reading! Even if you calibrate that out, it would change if you tighten the connections or if they relax, and it would change more than the tolerance of most shunts for a few degrees C change in temperature. 

It's possible to use Kelvin connections to surface mount resistors, for example. Some have 4 terminals, but you can split the ends of something like an 0805 to improve performance (not quite as good, but better than nothing).