you can reach more than 4.0 ghz with a liquid cooler. mine is currently stable at 4.6ghz @ 1.28v with air cooling (CM Hyper 212 EVO). I would shoot for maybe 4.8 if i were you. i wouldnt recommend bringing the voltage past 1.35v/1.4v. the max temp for that chip is around 77 celcius.
use prime95 version 26.6 to test your cpu. this version is required for this chip so make sure you get this version or you may overheat it.
http://windows-downloads-center.blogspot.com/2011/04/prime95-266.html
you can monitor temps with core temp
http://www.alcpu.com/CoreTemp/