Keeping increasing your voltage 0.01v at a time until you can get 4.2ghz to boot
In general, as long as you can keep temps looking good, you want to keep that CPU under 1.35v
It could well be that you've got a 4690K that doesn't OC that well, but trial and error will help find out
Also, might be worth reseating the cooler with a fresh application of thermal paste if temps start getting higher than expected.