can a plasterboard ceiling hold a 4 kg projector
While I hate mounting things in ceilings without reaching joists (or other framing members), some modern toggle type anchors can hold loads of over 100 kg each in 1/2 inch drywall per anchor (many jurisdictions require 5/8 or thicker drywall in ceilings). 

The load on ceiling mounted items is tensile and the use of several mounting anchors should give you lots of room. This link shows the load ratings of one brand of toggle anchors. Brand to brand ratings may vary slightly.

I strongly prefer the rotating bar type toggles over the old spring-wing types. They allow installation and removal without losing the interior component, and the load seems to be better spread over the drywall surface.

While two or more toggle anchors are theoretically more than enough for a 12 kg screen, the ratings are for static tensile use. If the screen is being moved up and down, either mechanically or by hand, that is a dynamic load that multiplies the strain on the fitting.

There still should be enough room for normal use, but understand that someone pulling down with their whole weight on a screen is more than any such mounting bracket can sustain.

Finally, things falling from the ceiling can do serious harm. Make sure that the projector, and especially the screen, do not pose a risk to humans (we can live with broken ceilings and devices).


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