The new map is open for all.
To get in there you have to go to Lorencia and talk to the NPC named "little_santa" located at the left of the fountain and buy a "Santa's Village Invitation". Than close the shop and right click on the item, than press Ok.
You will be warped to the new map in an instant.
The map does not contain a hunt area, so you wont be able to hunt for items / level up there, but that's the only place where you will be able to add options into your Season 4 sockets.
To add the options to your sockets you will need to talk to the NPC's below.