Does it give 100% guarantee that no data can ever be lost in case of problems with electricity?
There is no guarantee; there can't be a guarantee. There are too many variables.
1) is the computer plugged into a wall socket without surge protection, or is it plugged into a surge protector.
2) when power is interrupted, is it simply a loss of power or is there a power spike along with the loss of power.
3) if there is a spike without surge protector, how many (if any) components of the computer are damaged?
4) if there is a spike with surge protector, did the surge protector provide full protection, or did a portion of the spike reach the computer and cause damage?
And so on.
I live in a rural location. I have frequent power interruptions and occasional spikes. Most interruptions are short (one or more seconds) to a few hours (as during winter ice storms and power lines down). I have a surge protector plugged into the wall, and another on top of my desk plugged into the first protector. The one on my desk has plugins on the back for my peripherals. When storms move through the area, I shutdown and turn off the switches on the surge protector.
I have had to replace the first surge protector twice due to power spikes, but I have yet to have damage to the computer. Nor have I had data loss or apps suddenly becoming disfunctional.
I've used ext3 since shortly after it was introduced. If my understanding of ext4 is correct, journaling for ext3 and ext4 is the same.
So, the bottom line: can any data be lost using ext4? The answer is yes, with qualifications. See 1 - 4 above.