So let's get this straight. The timeline of the Insidious franchise goes like this:
The first Insidious is the third story in the life of its lead character Elise (Lin Shaye). Insidious Chapter 2 is the quasi-prequel/sequel to the first Insidious which acts as the fourth story in the timeline. Then the producers of the franchise seem to have thought "if it ain't broke, don't fix it", and hence we had a repeat with Insidious Chapter 3 which acted as the first adventure Elise has had, and there's now Insidious: The Last Key which is the quasi-prequel/sequel that we choose to class....