with the new 4.0 update my gpu increases 10ºC, I have less fps and it seems that everything looks very dark.
I personally would like to stay on 3.8, I hope they don't remove it.
colours in 4.0 are more realistic and darkness is...
No, that's just a false excuse.
Colors are
not more realistic in Odyssey, and the general darkness is
not the result of some artistic decision or "realism".
Tell me how it's realistic that planets are invisible until you get
really close, or how it's realistic that faint stars seem to blink in space if you steer the nose of your ship (scintillation is an atmospheric phenomenon).
Color changes (for example the color of the central star not affecting the color of things it illuminates) is just a bug.
Darkness is mainly caused by another bug (most probably some bad value in some image postprocessing step) which seems to destroy pixel intensity data close to the lower end of the histogram, that's why you cannot bring it back no matter how high you crank up gamma. It's probably the same issue what makes faint stars scintillate ("blink") as you move the nose of your ship, which is not happening in Horizons 3.8.
performance wise, as time goes on new things will need more power
Another false excuse. Odyssey (and Horizons 4.0) does NOT offer any kind of graphical improvements over Horizons 3.8 in legacy space gameplay scenarios (which is basically all what Horizons 4.0 has to offer), it literally only needs to render the exact same things in space (starry background, local stars and planets and nearby ships) so there is zero reason for it to be more power hungry. "New things" only justify the requirement of more computing power if it takes more complex computations to render higher quality images, which is quite clearly not the case in the Horizons 3.8 vs 4.0 relation.