Does this citation direct to the book Vergilius 4 published in 1940, on page 5-17?
If so, I am having trouble finding such a book written by Duckworth.
The full citation is: Duckworth, G. W. 1940. "Turnus as a Tragic Character." Vergilius 4: 5-17. This is the fourth volume of the journal and was published in 1940.
It seems the article is not available online. Vergilius took a hiatus (or had a different name?), and you're right that it's not on JSTOR. There's a collection of the early volumes on Google Books, but it only shows "snippet view" for me.
WorldCat lists three locations for the collected volumes, and if you have access to a library that offers InterLibrary Loan, that might be your best bet.
I found this is the Cambridge University Library catalogue:
Other Entries: Vergilian Society.
Published: Atlanta, GA: Vergilian Society, 1956-
Publication history: Began with vol. 1, (1956).
Description: ill ; 23 cm.
Notes: Supersedes Vergilius, no. 1-6, 1938-40.
Continues The Vergilian Digest, 1956-58.
Published by the Vergilian Society.