Regardless of whether the content you're seeking is found in the blue 4th edition or the red 5th edition, start with the Descriptive Word Index volumes, which are updated with pocket parts or supplemental paper volumes to include indexing of content from both editions.
Descriptive Word Index -- "Generally, descriptive words will group themselves around the five elements common to every case:
A search, in the DESCRIPTIVE-WORD INDEX, of words descriptive of these elements will produce the decisions dealing with similar fact situations and legal principles."
Table of Cases "lists alphabetically the title of each case, by both Plaintiff's and Defendant's names, the volume and page of the reports in which each is published, the subsequent case history, and the digest Topic and Key Number under which each point of law is digested."
Words & Phrases "lists alphabetically words or phrases that have been judicially defined in the cases indexed by the Digest, and sets out the headnotes, titles and citations of the case in which definitions appear."
There is no separately searchable file on Westlaw that directly corresponds to the print publication West's Federal Practice Digest. The print digest pulls together the headnotes from federal district and circuit court cases and arranges them by key number topics. Equivalent results can be achieved from searching Westlaw by key number, but this library research guide is focused on the print publication and does not explain how to use key numbers to search the online database Westlaw. Look for tutorials about Westlaw elsewhere, such as the Westlaw training resources.