how do I play a custom map?

Ok so I downloaded a .pk4 custom map. What folder do I put it in and how do i play it?

This depends on how you want it loaded. You can load it entirely seperate from the original game to prevent some serious screwups inside your "base" folder, or you can install it directly to your base folder and load it from there.

THE SAFER WAY:

Make a new folder in your Doom3 directory with whatever name you want, but don't put spaces in the name; then add the .PK4 to that folder. Next, load up Doom3, and select the Mods menu. Then, you should see an entry named the same as the folder you made. Load that mod, then (make sure you know the map name!) open up the console (Ctrl-Alt-Tilde) and type in the map name, make sure to include the directory relative to the Mod Directory! EXAMPLE: Map inside .PK4 is at "Mod.pk4/maps/omg/wtf.map". You type this in the console: "map omg/wtf", then your custom map should load up, unless a playerstart or a .PROC file is missing.

THE MORE TEDIOUS WAY:

Simply paste the .PK4 file into the "base" folder in your Doom3 directory. Then, load up Doom3, open the console and type the name of the map the same way you would with the "SAFER WAY".

EDIT: DO NOT, I REPEAT: DO NOT OVERWRITE YOUR ORIGINAL .PK4 FILES UNDER ANY CIRCUMSTANCES.

King of silly to have to do this. I hope they patch Doom 3 and make a browser that you can just pick the map from and run it that way.