(29 Aug 2014, 2:22 pm)G-CPTN wrote There are far more three-digit 'words' than four-digit 'words'.
By restricting the number of digits (to four) the 'computer' has fewer operations to make with fewer answers.
If you allowed a search on just one digit you would get a considerable number of answers, most of which would be false.
Two digits eases the situation, and three moreso, and four makes the computational 'load' tolerable.
Edited to add:-
SQL = Structured Query Language (Click) which is a special-purpose programming language designed for managing data held in a relational database management system (which is a table of words that constitute this forum).
What's a search throttle?!