Just a case of upping the multi mate. If it's not stable, up the vcore. Simple as that with 1155.
You could easily achieve a higher clock with a 2500k, though. 4GHz is still a nice overclock, just thought you should know that your chip could do more

Download Intel Burn Test, Prime 95, CPU-Z and HWMonitor.
Use Intel Burn Test on Maximum preset for 10 runs to check stability.
Then use Prime95 on its maximum heat preset to test for a good 24 hours.
If both pass you've successfully overclocked your CPU

If they don't, then up the vcore until they do.
Be sure to run HWMonitor whilst you stress test - make sure those temperatures aren't too high.