So 6V would damage the arduino?
No, operation would just be unreliable. The on-board voltage regulator would not be able to create a stable 5v. This would get progressively worse as the batteries start to die.
And the battery holder i have has 4 slots, so would the thing work with only 3 batteries?
As long as you replace the battery with some kind of jumper. Most battery holders expect a continuous circuit. An alternative might be to use rechargeable batteries. They tend to run around 1.2V per battery, so four of those would be very close to 5V.
And could i add a resistor to take away some of the volts?
That's not how electricity (and resistors) work. Resistors drop a voltage, based on the amount of current that flows through them. So as your Arduino project does different things, the resistor will be dropping different voltages. Additionally, the resistor is just going to burn (waste) power. Generally when you are running off of a battery, you don't want to just waste energy.
the battery pack uses 4 AA batteries, not tripple A
For the purposes of this discussion, that is irrelevant. They provide the same voltage per cell, just less power capacity.