1909, 1934, 1961 ... from this series, one might predict that the fourth edition of the Merriam-Webster Unabridged will appear in 1990. Alas, this is no Halley's comet. According to the August 1985 issue of Games magazine, The Merriam-Webster editorial director flatly state "there are no plans for such a work", and a noted lexicographer and editor doubts that there ever will be: "uneconomical...an unabridged simply doesn't sell enough copies". This article and the following one present two wistful (or, perhaps, wishful?) views of this unlikely work.
Wolpow, Edward R.
"Webster's Fourth: A Review,"
Word Ways: Vol. 18
, Article 4.
Available at: http://digitalcommons.butler.edu/wordways/vol18/iss4/4