Install wood panel in concrete ceiling
Four anchors are adequate if:

They're substantial enough to secure the load to the ceiling, such as lag screws with expanding shields
The plywood is rigid enough to handle the span (you could place the anchors at the 25% points lengthwise to reduce span)

Otherwise, use six or eight screws. It's hard to say more without more detail about your situation, such as type of concrete. Some general advice:

If you're able to drill 5/8&quot; holes, lag shields are a sure bet
If not, #12 or #14 screws in plastic anchors should do well if everything is sized properly
I'd use 3/4&quot; plywood. It's less likely to warp and will be rigid enough. You could get away with 1/2&quot; or 5/8&quot; with six or more anchors.

Followup questions from comments:

I didn't use any math. Math isn't great here because it makes all kinds of assumptions, many of which depend on your carpentry prowess. For example, if you use the wrong size screws in plastic anchors you'll cut holding power by 80%, and if you run screws too deep in plywood you reduce its hold by about the same. If the integrity of the concrete isn't there it's all out the window anyway. I'm relying on more than three decades of professional and personal experience, but  I don't have all the information I'd need to be specific.
I think four plastic anchors are barely adequate for a 30# load in tension (as opposed to in shear, such as with wall-mounted stuff), and I'd use at least six. Again, properly sized screws, to create the required internal friction, are critical.  Test well before putting it to use. If it doesn't hold you up, it's not done well.
Screw head type doesn't much matter as long as they aren't small-head trim-type screws. Good plywood won't allow them to pull through if you barely counter-sink them flush or leave them above the surface.
To reiterate, just four corner anchors may leave you with substantial sag in the sheet. I'd move them inward somewhat to reduce span.