So is 1.35V on water for i7 4790K safe? 4.8GHz

is 1.35V @4.8Ghz on a i7 4790K on water okay?

If the temps are good, then yeah.

at 100% its like 88C

A bit high for me, but anything under 90℃ should be fine for regular use, because you're probably not likely to max it out a lot.

It'll probably become unstable eventually, though.

What cooler are you using?

H100i

88°C on 100% load is a bit high.
However, how often do you realy put your cpu under 100% stress in the real world?

almost never. unless im using softwarfe mode on PCSX2 (because the graphics are better on software)

usually im only using 1 or 2 cores so yeah lol

then there is not realy much to worry about i guess. ☺

okay :) i was just worried that 1.35v in itself was dangrous

No i think its fine.
maybe you could try to lower it a bit, untill the point your system gets un-stable.
Then you know what the best voltage is for your chip at 4.8Ghz.

is it possible that some cores are better at overclocking than others? becuase it seems like core 1 and 2 can reach 5Ghz, but 3 and 4 can only reach at the most 4.9Ghz