Do i need to increase voltage?

Hardware Hub Overclocking
1

i overclocked my 4670k to 4.0 ghz and it booted and everything worked, but i've heard that you need to up the voltage. Do i need to do that

2

I would first stress test the CPU. If there is a crash or error, most likely.

3

I stress tested it for 5 min and nothing happend

4

I'd go for at least 20-30 minutes while tweaking. And when you're done, anywhere from an hour to a day.

Make sure you monitor temps!

5

Monitor temps, and run something like Aida64 or prime95 for about 30 minutes and see if anything goes wrong. If not, you don't need to change the voltage. You might even be able to up the multiplier a bit more before changing anything else.

6

DO NOT RUN Prime95!

prime95 artificially increase voltages messing with temperatures. This is not a problem for older cpus but for new (4xxx) intel architectures [like the one you have] and some amd architectures, prime95 is not a good way to stress your system for stability. It's great for stressing max temps because it cheats some things but it's not recommended with haswell.