I honestly don't know, but looking at it, I'm going to guess about 20,000 polys, with texture maps between 512x512 and 1024x1024.
Proper lighting and good texturing can give the appearance of far more polys than there actually are. A low level LOD can just be a flat poly with a 2D texture rendered on it but it could look hi-poly. That screenshot means absolutely nothing in comparison since we don't know anything about it.
FS2_Open can handle a lot of polys. The reason why you don't see many ships with more than 4,000 polys is that you might have 200 of those ships on the screen at the time. It's a game, so you have to make it look as good as possible, while still being playable. You can make a 100,000 poly model and it would work perfectly fine in-game, if there was only one of them on the screen at the time, and if you didn't have to render weapon effects, missiles, fighers, large cruisers, ship debris, and explosions along with it.