Reason for using 4- and 5-grams for co-occurrence analysis

Question

I am trying to replicate the results of a paper which didn't share its data methods that clearly. It makes use of the Web 1T 5-gram Version 1 data set collected from Google to expand 72 words into 964 by studying the co-occurrences of those words in 4- and 5-grams data set.

We expanded the original 72 terms to a lexicon of 964 associated terms by analyzing word co-occurrences in a collection of 2.5 billion 4- and 5-grams

Would there be any particular reason why they decided to choose 4- and 5- grams and ignore the uni-, bi- and tri- grams? Would the longer n-gram provide a stronger co-occurrence or maybe it allowed them to apply a weight on the distance from the target word if the window size included the whole n-gram?

Answer 1

What I failed to realise was the 72 terms were not single word terms but multiple word terms, from 1 to 4 words in length. With this understanding it makes sense to use 4- and 5- grams because some terms can't be used in lower n-gram collections.