I have the sensor mentioned above and it is working fine. One thing though and I wonder if there is a way around this. The doc’s say this includes a low-dropout linear voltage regulator which allows the sensor to be powered from a 2.7 V to 5.5 V supply.
Does this mean the sensor is regulated to 5.5V or do I need to use a smaller (4 cell) battery pack?
You should not power that board with more than 5.5V, so I recommend you use a 3 cell battery pack instead. If you use a fully charged NiMH 3 cell battery pack the voltage should be 4.5V, while a fully charged alkaline 3 cell pack should be 4.95V, both of which should work fine with that sensor.
The voltage rating on that 4 cell battery pack is based on the 1.2 V nominal voltage of the NiMH cells from which the packs are made. The actual voltage will depend on how charged the batteries are. A fully charged NiMH cell will have a voltage of almost 1.5 V, so that 4.8V battery pack can measure 6V after being charged. I recommend using a 3 cell battery pack or using a 3.3V or a 5V step-down voltage regulator to reduce the voltage of your 4 cell pack.
What if I want to use 4 of the Vl6180x in my carpet rover? Would I need 4 of the Step Up/Step Down regulators or could just use 1 step up/down regulator with different specs?
As long as distance from the regulator to the 4 VL6180X sensors is not a concern, you should only need to use one step-up/step-down regulator to power all 4 sensors.