to get to 4.2 shoud be realistic, since you have liquid cooling. ive never oc'ed an i series before, but i think that if you increase the vcore to the 1.38, and increase the base clock from 99.8 to 129, you should reach 4.2ghz on your cpu. thats a 30% oc, and many mobos will take it, but some may not run at a 30% increase in base clock speed. im not familiar with how well gigabytes boards overclock.
you might want to disable turbo boost and intel speed step within your bios.
well, after researching your board, i found an overclock test on exactly your mobo/cpu combination. apparently, your cpu has an unlocked multiplier, so you dont have to increase the base clock by 30%, you just have to increase the cpu multipliers for each core from 33 to 47 (leaving the base clock frequency at 99.8mhz), to increase the chip from 3.3ghz to 4.713ghz, increase the cpu vcore voltage to 1.404V, use 1600 speed ddr3 ram, with an 800mhz memory base clock and a 24:3 mobo to meomry base clock ratio, and 7-70-20 click ram timings. heres a link to the article.