you can use func_reflective_glass. Apply a special texture that uses a lightmappedreflective shader. Some stock ones can be found by searching "glass". There are 2-3 shattered ones and 1 normal i think.
To make a 4d skybox: -make your surroundings. -make a sky_camera entity at the map's origin -select the skybox stuff and the sky_camera -scale them down 16 times, to do this press ctrl+M and type a value of 0.0625 for XYZ in scale. -move all this into a seperate skybox box.
if you want to edit the skybox later, you can scale everything up 16 times, and align everything so that the sky_camera is at the world origin again. Then you can do everything fullscale, and scale down again.