My second density cat can see and touch me.

[Unbound]

So a whole density is capable of making a collective choice or is the choice made through consensus of all entities of those densities?

Not the whole density. Just the portion of 4th density associated with whatever 3rd density environment it is attempting to shield itself from. I would imagine the social memory complex of that 4th density planet reaches a state of collective discipline and choice of not lowering their vibrations to the point where they may become detectable by any 3rd density lifeforms.

So why dont we see all the various forms that supposedly do not make this choice? Do you think that any being of anothet density could instantly make themselves visible.

Although 4D is treated as distinct from 3D, they both are always existing.