Well I definitely wouldn't feel safe going past Intel spec of 1.55 Vcore and 1.35 VTT/QPI/IMC (whatever your mobo decides to call it).
1.264V is nice and low, and so is the temps. At 4ghz, that's pretty awesome. Personally I wouldn't worry till at least 1.4V Vcore and 1.25V QPI. If you can use C States and/or C1E, EIST then even better as it'll reduce the idle power used.
But yeah, you've still got headroom.
Unfortunately as for "running 24/7" with, say, 1.45 Vcore and 1.3 QPI, nobody really knows how long it would last. Having super low temps would definitely help a ton but you can still get electromigration. So lower is better but if you have a target speed to reach, go for it and if you feel the voltage is just too high for a permanent OC then back it off a bit. I'd like to know how high you can take it.
I'd just like to make a suggestion. Try pumping more Vcore to keep from having to increase the QPI. At least on my rig, I find they seem to work together so you can make up for one being low by boosting the other higher. You're at the QPI limit from Intel, so yeah... I'd look at only adjusting Vcore at this point.
But 4.2ghz! My 750 doesn't seem to want to go that high... At least not without a lot of voltage.
Only mananged to get the QPI down to 1.335V, 1.315V wasn't stable, gave me a BSoD after 15 passes of LinX, testing Vcore at 1.328V now. Max temp is 62C so far
4.4Ghz testing tomorrow, I have a feeling alot more warning lights will be appearing on my mobo