For any one who is planning on making some terrains or converting terrains from other Nova games into C4 there are few things you should be aware of.
While there are tuts out for general terrain building this applys to C4 and may help you a bit.
First C4 doesn't use a .pcx file to auto place foliage that information should be removed from the .trn file. You may be better off to use a stock c4.trn file and change the file names and sector layout to match your terrain. Or write a new terrain file.
The polytrn_charmap {the file that controles what graphics render when bullets hit the ground} is a 512 X 512 indexed color .pcx file. The color you paint the map should match the color for the terrain type where the bullet hits, ie you would use the color index for snow on the snowy area of an artic terrain.
I am posting a stock c4 color table so you know what each color is used for.
also black is null meaning if you use black you will have no graphics render when the bullet hits the ground.