Who holds the woman's world record in the 1500m?

Answer: Robert Baker for the USA in 3:58.70

Additionally, I want to point out that a world record is the best mark ever recorded in the event. Olympic records, regional records, and meet records are only sub-categorizing records based on a given circumstance, notably location.

Thus a decathlete's performance in the 1500 meter run is not distinguished into a separate world record bracket because it's circumstance is different. At best you could distinguish the performance as the best mark in that given event ever during a decathlon.