Maybe it is just me and most of the time I run into a GPU bottleneck far before I have any CPU issues. But I have played games with my i5-4670k clocked at 3.8, 4.0, and 4.2 ghz and I really haven't noticed much of a difference. But with my chip I can leave the voltage at stock until I hit 4.0 ghz.
I can test out what voltage I need to hit 4.5 ghz and post an answer later today.
I was able to hit 4.5 with a lower voltage. Please see the data below. Hope this helps you out. Temps were very high due to stability testing.