In case anyone is wondering, volume five is "forthcoming":

See: https://geography.wisc.edu/histcart/

and: https://geography.wisc.edu/histcart/volume-5-cartography-in-...

Only volume 1 came out in 1987; volume 2 was published in three books from 1992 to 1998; volume 3 came out in 2007, 6 in 2015, 4 in 2019.

academic publishing allows for out-of-order execution.