Minor Cleanup and Further Extension of Built-In 4- and 5-Grams

The number of AW built-in alphabetic 4-grams has reached 2,020. This probably will keep rising, but for philosophical and practical reasons, an AW upper limit for 4-grams has been set at 2,500. It is getting harder to find hundreds more alphabetic 4-grams that are common enough to make much difference in n-gram indexing.