angelo143 :
With my 6700k I could overclock to 4.5ghz with a 240mm rad. If you lucky and get a 240 or 280mm rad you may hit 4.6ghz. I am not fan of the air cooling . So I would recommend you buy a water cooling.
Don't forget to mention the danger that can be included with water cooling of any kind. Leaks. If it leaks, you could be in trouble. TBH, sounds like your 6700k wasn't a silicon lottery winner. Air cooling is using just as efficient as AIO for most part.
Usually only custom loops yield better temps than a air cooler ever could but that's also a lot of $$$.