Answer
The diagonal is about 5.6 inches long.
Work Step by Step
D$^2$=4$^2$+4$^2$
D$^2$=32
D=4$\sqrt 2$$\approx$5.6
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.