I would be amazed if you managed to create a map editor for the 4th generation.
I kinda wish I was a programmer, then I could've been able to help and all that.
A 4th generation map editor would have to require reading the models and textures from the ROM. If I am correct... the map editor would also have to require the abbiliy to drag the objects and stretching them. And of course using the textures on the polygons.