Is the dead garden mask only free for pc? How about PS4?
The Deathgarden mask is exclusive to players who own both Deathgarden and Dead By Daylight. Since Deathgarden isn't available on consoles, the mask is currently available only to PC users.
Hopefully you'll get it with the exclusive items update
It wasn't on the list of exclusive items that they said they were granting, so unfortunately I doubt very much that that will be the case.
An update to this, DeathGarden was scheduled to be released on PS4 around 2019...
But since DeathGarden is actually being shutdown around the end of this year, this is no longer a possibility. You could say they'll do something similar with the exclusive skins and just release the skin for everyone after DeathGardens passing...
This isn't an official confirmation, so please don't take it as one. It's unknown what they'll be doing with the skin after Deathgarden is officially gone, but we'll just have to wait and see.
Hypothetically we'll say this isn't happening, if that's the case than the game was supposed to be scheduled for release on PS4 and once purchased would gift any players that own the copy of the game with the DeathGarden mask.
Of course, that's hypothetical. In reality this isn't going to be the case, and it's unknown what's to become of the skin. (For now.)