Basics are the same. Just some key differences. Unlike UE3, you must start with a subtracted cube and then add actors/additive brushes into that world space, You can't just add into the "Void". Also, unlike UE3, there are skyboxes.
Unlike UE3 where you just add a huge dome over your map, skyboxes are a separate cube/zone away from your map. It is usually also miniaturized(Usually no bigger than 2000uu). From there you add in sky textures, sky domes, what ever you want as a back ground. You then add a skyzone info into the skybox. After that you may set textures in surface properties to unlit/fake backdrop and the skyzone info acts like a camera that projects what it sees in the skybox to all the textures you set to fake backdrop.
Staticmeshes/Textures/Terrain/Sounds are essentially the same but they are not packed into .upk files. They are all divided into their own folders in the game dictionary.
Animations: .ukx
Maps: .rom(.ut2 for UT2004)
Music: .ogg
Sounds: .uax
StaticMeshes: .usx
System: .u
Textures: .utx