In-ceiling speakers: box worth it?

Building a home theater, including a "drywall" ceiling. I've decided to mount the speakers in the ceiling. They are going to be small ones, perhaps 4" FaitalPro 4FE32.
And I started wondering. Can I simply install them and add some soft material behind, making it an "infinitely large box", or should I do (vented) boxes?
Boxes would require quite a bit of work, but would it be worth it? Is there any benefit, sound quality-wise?

Edit: dang, turns out I should be celebrating 20 years on diyaudio 😵
 
Sorry for noob questions, but my intention was to use these as satellites and center/voice speaker in a 5.1 system. Or should I look for something else? I'd like to stick with 4" though, max 5" + a large sub. The room is 50m2 / 500sqft and will have a 5" high "step" made of gypsum/drywall around the ceiling, so the drivers will be side-firing, not downwards, like in the usual ceilings.
 
Your goals are likely conflicting.

The 4FE32 will have very limited output capabilities at low frequencies (this is simply a function of volume displacement - Sd x Xmax). Since you mention a large sub, I assume you want more output than a high sensitivity 4-inch can provide in this scenario.

Larger full-range drivers tend to beam at higher frequencies. If I'm understanding your layout intentions, you will be way off axis. A co-axial or a different physical layout would probably make sense. Even a car co-axial speaker could be used if you want to keep the cost low. Most of those are designed for infinite baffle conditions.

If you have a small seating area, maybe you could deal with full-range beaming through equalization alone, but I'd test it before committing to the build.

Angling your "step" so the drivers face the listeners more would help if you really want a full-range speaker.