Why does this analog front end EVM have 4 pins per channel, instead of 2?
One entire column of the pins are just ground.  They exist for the use of the jumpers.  The jumpers should be installed:


On the - pin of any channel where you want to make a single-ended reading instead of a differential reading
On both + and - pins of any channel which is not actively used.


The former is to set the non-inverting input to be at ground, thus making it essentially a single-ended ADC.

The latter is to tie both inputs of that channel to ground so they aren't a floating input.  A floating input can potentially induce noise into the ADC affecting other readings on other channels.  By tying the inputs to ground you are reducing that noise to a minimum.