Great explanation from an IRC talk I've had in the past with SwK:

<SwK> master and 1.5 are dev code
<SwK> 1.4.x is release code
<SwK> FS version numbers are pretty easy to figure out
<flokr> damn, so should I run 1.4.X instead?
<SwK> 1.X.Y where X is even thats a release branch, where X is odd thats a dev branch. Y is just the last tag.release point
<flokr> wooooooot, SwK, it's so clear now
<SwK> now where X is odd, X-1 is the release branch from that dev code
<SwK> example 1.5 dev/master branch feeds the 1.4 release branch