So this four-digit number is decomposed into the product of prime factors = x 6 * y (it can't be x13, because 213 >; 2000)。

X cannot be a prime factor with the last digit 1 (because the minimum prime factor 1 1 make116 >; 2000),

X can only be 2 (because the prime factor 3 makes 3 6 = 729, and y takes the minimum prime factor 1 1 the last digit 1, which also makes 3 6 *11= 8019 >; 2000).

If y= 1 1, then 2 6 *11= 704 is a three-digit number.

Y=3 1, then 2 6 * 31=1984 is exactly four digits less than 2000.

And y=4 1 is 2 6 * 41= 2624 > 2000,

So this four-digit number can only be 1984.