[Admins]

You use the level generator to generate the gmap and levels (nw), each level is 64x64 pixels so you need a 256x256 pixel bitmap for generating a 4x4 map. The image can be completely green color.
After generating it you open the levels with the level editor and put the right tiles. You can drag&drop the tiles and right-click on the level to fill it with the selected tiles (can be zone-grass or whatever) if they are all the same tiles, although I am not sure if the level generator makes "clean" levels with just one single tile. If there are problems then just do the normal way of filling it with tiles by right-click copying.