FPS Decreases when using Unity4, why?

Hello everyone, I have searched a lot and tryed a lot of different ways to solve that problem but nothing works.. so what's my problem. I have made a map with Cinema4D, it's medium sized map. When I import it to Unity3 it works like charm. FPS above 40 at around 60-70 sometimes even better, it depends on how much I use the system at the time I play with the editor. But when I try to put the map in Unity4+ it gives a lot of lag. FPS Drop which is very annoying. The FPS becomes 30- at around 20-10 FPS .. Occlusion Culling is not working, LoD is also not working, FullScreen/Windowed not a solution too .. can someone tell me what causes that FPS drop and how to solve or just to continue working with Unity3 ? Thanks in advance!