If Zoey has an item that giver her an extra two dice during combat and she did not lose any sanity during a combat encounter does she get to roll 4 additional dice during combat?
Zoey samaras
Yes, additional dice are cumulative.
From the first part of reference guide p: 11 for tests (text in red for what applies here):
When an investigator resolves a test, he performs the following steps:
Determine Dice Pool: The investigator determines the number of dice he will roll. This number is his DICE POOL
• He starts with a number equal to the skill being tested.
• He adds or subtracts the test modifier (the number that follows the skill icon on the effect).
• If he has an Improvement token for the skill being tested, he adds the Improvement token’s modifier.
• He may use one effect that provides a bonus (for example “Gain +3 {will icon}). If multiple effects provide a bonus, he uses only the highest bonus.
• He adds any “additional dice” he gains from effects that apply. Additional dice are cumulative and are in addition to other effects.
Thanks!