A quick glance at outstanding issues will show a number of integration bugs are still outstanding, and the new v8 was landed only a few days previously - incompatibilities with new versions of v8 can take a while to surface in node.
It's important to remember that semver makes no promises about stability. While we're used to "N.0" meaning "stable and ready for upgrade," that's a non-semver idea that is explicitly disregarded in the semver release model. Node moving from 3.* to 4.0.0 only indicates that there are breaking changes(1).
If you have concerns about stability, you should take signals on that from the node LTS group, and pick LTS releases.
1 - whether or not this is good, bad, or merely a different permutation of the things that are turning your hair grey, I leave as an exercise to the reader.