Stable, temperatures below 75c: 4.6ghz.
Stable, temperatures reaching 95c: 4.8ghz
Stable, getting to the point of throttling in stress tests: 4.9ghz.
Highly unstable, but enough to do light benchmarks like SuperPi: 5.0ghz.
What I'm actually using is 4.0ghz (turbo disabled) for general usage and switching quickly to 4.8ghz when rendering large files.
All this on a $30-$40 air cooler, by the way.
Watercooling being superior to air cooling is only a myth (not even true), by the way. High end air coolers like the nh-d15 cool as good as a H100i, are not even half as loud (12dba less under load) and cost less on average.