1.4V will be the max I'd try on ambient cooling.
What is bad for your CPU is the current draw. You can't monitor current draw directly, but voltage plays a big role on current draw so that's why people use the term "safe voltage" instead of "safe current". The max rated current for 84w Haswell parts (including the 4670k) is 95A, so if you really want to try your luck without breaking the CPU, set the CPU core current limit to 95A and start pushing voltage. It will throttle you if your current draw exceeds that limit.