All four terms refer to a specific date before which the carrier won’t cover claims, but they work in different ways:
Retroactive Date, Prior Acts Date and Continuity Date all essentially refer to the same thing. Any act that occurs before this date will not be covered. Even if the claim isn’t actually made until halfway through the policy period, the carrier would deny the claim if the act that generated the claim preceded the specified date.
Pending & Prior Date refers to pending or prior litigation. The focus here is on when the claim is made, not when the act occurred. As long as the insured first receives the claim and makes it to the carrier during the policy period, it doesn’t matter when the act that generated the claim occurred.
Important note: the meaning of these terms can vary between carriers. Make sure to double-check the policy language.