There isn't any such option on Obsidian v4. I think it would be nice to have a fade in/out option, where the intensity of the lights increases/decreases respectively when activating/deactivating. I've seen some sabers that do it, but I think it was either a custom job or else manufactured by the "other guys." As for this being on Obsidian, which deals primarily with sound, I don't see that as a practical addition. Now for Emerald, that would be something else entirely. Emerald v2 with an activation intensity setting? I would like that.
One other idea that I had to sort of fake it is to work with the blade flicker on Emerald. Have the first color transition start as completely black, then take a few seconds to fade it to your blade color. Set the hold time on that blade color for a long period of time; 2-3 minutes, you know. You'll certainly have a clash to reset that second blade color and start the time over. With this, you would at least get a fade in. Not a fade out, but a fade in, which is better than nothing I suppose.
I have a couple of hilts with non-Obsidian soundboards and those soundboards have a shimmer setting you can configure. It adjusts the amount of juice to the LED creating the flickering effect. I was just wondering if the new O.v4 has a similar feature. I assume it does not since it was not mentioned in the video or on the site. Both of my hilts are using a standard LUX LED.