Why are these called Volumes 1-4? How does this align with the Level and Part system of the 3rd Edition?

The four-volume series has been renamed to better reflect the flexibility of the materials and diversity of our user groups and their instructional environments.

Volumes 1 and 2 correspond to Level 1 Parts 1 and 2 in the 3rd Edition, and Volumes 3 and 4 correspond to Level 2 Parts 1 and 2. In recognition of the fact that Volumes 1 and 2 are often used together in the first year of language instruction, and Volumes 3 and 4 in the second, the sequencing of lessons from 1 to 20 in each half of the series has been retained.