I want the light to fade from very bright to dark, dark and finally no light. Since the ambient light is not adjustable, how do i achieve this? Do I need to fade to black?

( Use a shallow compressed dark mist in the background distanced just beyond the 50% mark of the set if a night scene. )

Use all four lights to create a sense of overall lighting (or however your concept re: scene elements demand) and in LIGHTS > MODIFY > select ANIMATED LIGHT > EDIT KEY and edit them at the stage you need them extinguished.

I'm assuming you know that
if at frame 1 a light is set at its brightest
and at frame 2000 it is set to about 5% from being off,
and then at frame 4000 set again to its brightest,
the movie will show almost pitch dark in a slow fade out at frame 2000
and then bright again slowly at frame 4000.

This will give you a drop out of lighting to zero.

W.I.P. "Medieval Chinese Painting Exhibition" over 6 sets.
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No Imagination, no comment!