i will show you how to create a Under/Overgrowth Environment in about 5 mins (also 4km Maps)
you need bf2_tpaint ( or a coloured map Layers 1 - 3 )
open up your map and go to the terrain editor and select the Undergrowth-Tool (1) create 3 materials and use the first 3 coloured buttons  ,  and  or it will not work (2)
add some stuff to the materials (not neccesary but it looks better in my case)
press the Import DetailLayers button in the Toolbar (3)
now it should look something like this (Render-> Undergrowth Mode)
go to the Overgrowth-Tool now add also 3 materials and use the same coloured buttons like you did before in the Undergrowth-Tool ( ,  and )
add your trees and bushes to the materials and save !
DONT CLOSE the editor yet just go into your mapfolder and select your Undergrowth.raw and copy it then go into the Overgrowth folder and delete the Overgrowth.raw and insert the undergrowth.raw rename undergrowth.raw to overgrowth.raw
go back into the editor and reload your map now it should look something like that
and thats it ! really easy not ?
this is my first tutorial and i dont have any xpierience with that so if you dont understand sth just ask
and the last: one of my 4km Maps more than 200k trees @ the beginning and now around 63k trees (still alot)