Hi i have the same cpu. i done some tests for a friend and found that at stock or 3.4Ghz the intel HSF is perfect. but if you motherboard like mine enable cpu boost (the cpu jumps from 3.4 to 3.8Ghz) the stock intel HSF might strugle a little bit.
when the i5-4670K hits around 85'c it will start to throttle down (run slower) to help cope with the heat. so this leaves you with 3 options. first is to live with it running hot. second is to disable cpu boost so the cpu keeps at 3.4Ghz or third is to get a good HSF. you don't have to go crazy. just someting that can handly the extra heat.
Oh this is assuming you computer case has good air flow and is exhausting the hot air ok