You can set it higher than 3.9. 3.9 is literally what your max speed right NOW is. like right now, your PC is set to have a max speed of 100x39=3900Mhz or 3.9Ghz.
100x42=4200Mhz or 4.2 Ghz
Your chip can EASILY push 4.6Ghz but i didn't suggest a number that high because it can cause instability - and you're not familiar enough with OCing to combat instability.
As long as your CPU isn't going over 85degC you're totally fine.
You have a new, common chip. Literally just googled "overclocking 6600k" and read the thousands and thousands of correct and current articles out there on it.