The limitation is a result of MySQL's FullText indexing. I could reduce the number of required characters to 3, but it would make the site veeeeerrrrrry slow when even a single person is searching.
The next upgrade is due out very soon. They're in the last stages of testing before releasing it, and I plan on getting it implemented this summer. It will be a major upgrade for us, since we skipped the last one. The last one just didn't offer enough improvements to justify the effort needed. This version, though, is a complete rewrite from the ground up, so I'll start working on it as soon as the final version is released.
I'm not sure if the new version includes improved search or not, but there was talk of using an open-source, 3rd party indexer to handle the searching. If it is included, I will definitely implement it.