2 Oct 31, 2010 19:54
Welcome to the forum.
You can install captcha plugin in any version of b2evolution. Basically if you are satisfied with v2, if performance is not a problem, and you do not need new features, the upgrade is not needed.
Upgrades usually go flawlessly if you haven't changed the database manually. v4 is the most recent and feature-rich, it handles comments better than any previous version.