Not really dangerous if you know what you are doing. Will it shorten its life? Perhaps. Depends on how far you take it and what cooling you have. 4 GHz is modest number should be perfectly fine, just get an aftermarket cooler, like Hyper 212+. Technically, you should loose warranty, than again, if you don't try something extreme, your cpu should be fine and you won't fry it and nobody would be able to tell that you overclocked it.