Rob, the standard definition of "normal" focal length for a format is the format's diagonal. It is pretty universally accepted for formats larger than 24 x 36. Cine-camera formats and derivatives -- 24 x 36 is double frame 18 x 24, the classic "35 mm" cine camera format -- use arbitrary definitions that bear no relation to the format. By convention, 24 x 36's normal focal length is 50 mm even though the format's diagonal is 43 mm. Except when the SLR manufacturer can't make a fast 50 mm lens that will clear the mirror, in which case "normal" is redefined as 58 mm. Whence the 58/1.4s "normal lenses" for 35 mm SLRs of the late '50s through mid-60s.

And then there's Humpty Dumpty. " “When I use a word,” Humpty Dumpty said, in a rather a scornful tone, “it means just what I choose it to mean—neither more nor less.” That's you, I think. Ignorant or willfully idiosyncratic.

To get back on topic, 4x5's diagonal is approximately 150 mm.