Major differences from a 4 to a 5 are the nozzle and the PEI bed surface. The power supply is also better in the later Taz 5’s.
From a 5 to a 6, the main differences are 12mm x rods, integrated PSU housing inside the control box, UL listed, Different feet, auto bed leveling (mini style) slightly different leadscrews and motors, the fillament guides are capable of folding up instead of fixed, and the overall print area is minimally smaller due to the auto leveling space requirements. (about 3mm smaller in one direction if I remember right). Also the Carriage and hot end are much better, no more stupid squirrel cage fan, two PLA fillament cooling fans, more rigidity, etc.
The larger rods, carriage and the auto leveling are the biggest features.