All cores boost to 4.3GHz and only one core at a time will boost to 4.7GHz.
Some motherboards have advanced turbo settings that allows all cores to boost to 4.7GHz, which does require a good cooler and a $50 cooler may not keep it cool enough.
[strike]But for $50 I'd recommend a Cryorig H5 if your case can fit 160mm tall CPU coolers. [/strike]
*Just saw your case, it supports up to 155mm. Looks like you'd be running stock speeds until you got a better case. Honestly though, the 8700K is a great CPU at stock speeds.
Just get a Cryorig H7 (or H7 Quad Lumi, it has an extra heat pipe) for now, and later if you want to upgrade case and cooler you can do that.