Version 4.6 is the latest RELEASE version; it was released in June, 2002. This is also the latest STABLE version.
Briefly, -STABLE is aimed at the ISP, corporate user, or any user who wants stability and a low change count over the wizzy new features of the latest -CURRENT snapshot. Releases can come from either branch, but -CURRENT should only be used if you are prepared for its increased volatility (relative to -STABLE, that is).
Releases are made every few months. While many people stay more up-to-date with the FreeBSD sources (see the questions on FreeBSD-CURRENT and FreeBSD-STABLE) than that, doing so is more of a commitment, as the sources are a moving target.