I have a model of the Abyss Cab 3 Mini sub. The model has a number of spotlights that cast a beams in a volume. It all works in 3.6 but in 4.0 all my spotlights are broken and don't work like the old spots. How do I get the 4.0 spots cast a beam rather than a circular glow that encompasses the whole fixture. this is not the way spot lights work. They cast a beam in a single direction and virtually no light in the opposite direction. 4.0 breaks every undersea model I have because of this lighting change.
I originally voted to close this question as rather being a bug or even intended behavior which makes this question off-topic for this site.
In the meantime I found out the spotlight is actually intended to work like that now and at least in the 4.0 Blender versions there will be no change to it. The googd thing about the new spotlight is, it respects the given radius better than the old one.
However, for everyone who wants to replicate/substitute the previous spotlight behavior, here is a workaround:
Instead of using the Spot, change the light to Area instead. Change the Shape to Disk and the size (since this represents the diameter now, not the radius) to twice the size as it was before. Under Beam Shape, the previous angle for Spot Size now needs to be set as Spread for the area light (the default of the Area light is 180°).
Unfortunately the area light seems to be much much brighter than the spot light. In the volumetrics, a Power of 30 - 50 W seems to have a similar effect like a Power of 1000 W on the area light.
So this is not a perfect equivalent of the old spot light, but at least a usable workaround.
Here are some comparison images, first of all the previous spot light in older versions:
And this is what you are experiencing now in 4.0:
This is what you can get with an Area light, as you can see it is much brighter:
This is the same with a reduced Power value of 40 W: