4km jungle or forest maps are pretty tricky to say the least.
There is no real limit on the amount of overgrowth objects you can have per se, but at some point, can't remember the exact number, if you have too many OG objects then not all of them will cast shadows on the terrain. This however can be overcome in a few ways, best of which would be to import all the statics, overgrowth and terrain into 3DsMax and do the lightmaps in there but that is pretty technical.
On top of that you have the issue of performance, with if you have lots of trees in a players view, then that is going to decrease FPS for many players. Simplest solution to this problem is to lower the View Distance of the map but in doing so you can't really have aircraft on the map, maybe choppers if its not too low but anything under 800m you can say bye to fast jets, which ideally need over a 1km VD to function well. Choppers its something like 500m minimum but again, the higher the better for both the choppers and the players on the ground trying to shoot them.
Another simple solution that is normally taken is to make the jungle/forest seriously thin but having each tree 10m apart looks pretty dumb and not like a forest at all. You can also have lots of clearings to allow areas where you have trees to be more dense but too many large clearings and for a jungle anyways, its not going to look very good either.
There are a few tricks you can use to increase performance, the density of your jungle and the VD of the map and this would be to only use the large to medium trees in your overgrowth to create the main canopy and emergence layers. Then to fill the under under canopy and shrub level / forest floor, you used the small trees and bushes as just normally placed objects, mixed in with quite a bit of undergrowth too to add even more close up detail, but also on top of having all the small trees and bushes placed as normal objects, you would need them coded to "cull" pretty quickly for the most part so they only drew upto a few hundred meters, depending on there size so you would only render them once you are basically inside the forest. You would also not have to technically place them by hand, you can place them in a few ways, simplest is with the overgrowth tool, separate from your main trees, and then convert them normal objects although you will need to watch out for bad placement, mainly with the small trees growing inside/through the large trees. You would also want to do a second overgrowth layer with medium to small trees/bushes in it, possibly a few large to go around the outskirts of your jungle/forest edge where the trees would generally be smaller anyways, so that when players peek into the jungle from outside it, they don't see an empty under canopy due to all the objects inside placed as objects being culled really quickly, but you would only use this around the edges of your jungle and inside all the objects would just get culled quickly
I've yet to see anyone attempt the last idea of making the under canopy normally placed objects since its kinda tricky to pull off but I believe this would be the best way to make a jungle or forest in BF2 with the best performance.
(big version here: http://i.imgur.com/FyD5sJi.jpg)
(big version here: http://i.imgur.com/jUfQyNp.jpg)
. I made some tiny changes, so images now do show up.
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R now)