The cables are twisted to make the loop area smaller and to cancel out magnetic interference. They are shielded to block noise from electric fields.
There are a few ways noise can enter a shielded cable and create noisy currents. The main one that you need to worry about is called mutual inductance. When two parallel conductors are next to each other, the magnetic field from one can couple to the other conductor and create noise. This is going to be less of a problem with twisted pair (because of the magnetic cancellation of twisted pair, but it doesn't cancel everything)
In terms of cable selection should I use a cable with each pair
individually shielded or go with both pairs shielded as a whole?
It depends... there will be crosstalk between the cables if they are all in the same shielded cable. There isn't a lot of information on how much crosstalk, and it would be easier to get some cable and measure the cross talk than to calculate it (IMO). It depends on how worried you are about noise. If you are designing for cost, I would go with one cable with four conductors, if your designing for low noise, I would go for four shielded cables.
It looks like your not pushing any boundaries with length\data rates...