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.