The only reason I could see they did that is possibly because of the time it would take to search the database for anything starting with just one letter. 4 letters allow the results to be more concentrated.
You can homeroll your own predictive search using a repeating group (placed underneath the input) and a regular text input. If you set the repeating group to search for data containing the input…it will begin filtering from the first letter.
Lots of Bubblers have used this approach for various reasons.