You can push past 4GHz with stock cooler if you have a good chip. It's all about the temperature and voltage limit you set for yourself and the quality of the silicon in your processor. You shouldn't view it as "I need to get this overclock" but rather how much can you overclock within your set limits and the cpu on hand. This said it's impossible to answer your question definitely. You'll have to just try and see. I don't think the cooler is going to affect you that much though.
Less is more. However it doesn't make that big of a difference (not as much as people usually make it out to be).That much really? doesnt seem like very much? Maybe thats why my rig sometimes has heat issues when streaming.....