4' double fluorescent lights fixture
Drywall anchors are used in vertical walls and the rating is based on a combination of shear load (downward on the screw and anchor) and some tension load outward (based on an assumption of a load close to the wall).

Ceiling mounts are pure tension loads downward and the weak spot is the drywall, not the anchor. A thin diameter of crumbly plaster around the anchor is all that holds them in. Even flared, they form a narrow cone that is easily compromised.

For ceiling mounting, try very hard to find framing members. If that is not possible, use anchors that have significant perpendicular members, such as molly bolts or toggles. I favor strap toggles like these



But I really urge screwing into framing if at all possible.

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