The 4-way is the one people misunderstand. It does not block anything. It takes air that is dropping straight down out of a diffuser and pushes it sideways across the ceiling in all four directions instead.
That is a real fix for a specific complaint: not a draft from one side, but a cold column falling directly onto whoever is underneath. Spreading the discharge horizontally takes the dump out of it.
Standard body: 24 by 24 inches, 2.75 inches deep, polystyrene, held on magnets against a compatible steel grid or diffuser face. Since every side is open, it goes on any way round.

What does a 4-way diffuser cover do?

It redirects the discharge outward across four sides rather than letting it fall straight down. The volume of air does not change and no direction is closed off. What changes is the shape of the delivery: a column becomes a spread. If the complaint is that air drops onto a desk directly beneath the diffuser, this is the configuration aimed at exactly that. Nobody underneath is being shielded, but the air arriving on them is arriving sideways and slower rather than straight down and cold.

Is a 4-way cover the same as no cover at all?

No, and this is the question worth answering clearly because the assumption is common. A bare diffuser has its own throw pattern, often with a strong downward component depending on the type and how it was set. The 4-way cover puts a physical deflector in that path and turns the discharge horizontal across all four faces. The air still goes everywhere, but it leaves the fitting differently, and that is the whole point. If your diffuser already throws well horizontally, this configuration will change less than you hope, which is worth knowing before you order.

When should you choose 4-way?

When the problem is directly underneath. A workstation that cannot be moved from below a diffuser. A treatment chair, a till position, a desk in a fixed floor plan. The 4-way is also the gentler choice when a diffuser simply runs too strong and you want the same air delivered more softly over a wider area. What it will not do is protect one person while leaving the rest alone, because nothing is blocked and no direction is privileged. For that you want the 3-way, which closes the single side causing the trouble. The 4-way is about softening delivery everywhere rather than shielding anyone in particular.

Will it fit your diffuser?

Three checks, same as the rest of the family. A nominal 24-inch-square diffuser, which is the standard ceiling grid module. At least 2.75 inches of clearance below the ceiling line for the cover's depth. And a steel attachment surface, because it holds on magnets. Aluminium grids and painted plastic faces will not take it. Hold a fridge magnet against the surface before you order.

How does it install?

Easier than the others, because there is no orientation to get right. Lift it into place and let the magnets take it. No tools, no fixings, nothing to make good. It lifts off by hand just as easily, which makes it straightforward to try, move to a different diffuser, or take away entirely if it turns out the complaint needed a different configuration.

Does this affect the rest of the system?

Very little. No side is closed, the outlet passes the same volume, and the air still leaves in every direction. What you have changed is the delivery pattern at one fitting, not the distribution across the branch. Of the six configurations this is the one least likely to move a problem somewhere else, which is worth knowing if you are treating several diffusers on the same run.

Is this a filter?

No. It changes the shape of the airflow, not its quality. No filter media, no MERV rating, no filtration claim. Filtration covers are a separate product.