I came across the anouncement of the Band IV of Altindische Grammatik of Wackernagel-Debrunner, Verbum und Adverbum (in Vorbereitung), in the Register volume for the first three volumes published by Richard Hauschild in 1964 (on the back of the cover page).  Was this volume dealing with "Verbum und Adverbum" ever published?  I have looked around, but not found such a volume.