What are they?
BTW: Can i use poser4 models in poser3?
im not sure, but i could send you a poser 4 model if you want. im asuming your asking because you have 3?
im getting it from the Digit magazine for free 
Poser 4 characters based on the new figures included with the software won’t work in Poser 3 because you won’t have the geometry files in Poser 3 to open them. It’s not legal to distribute the obj models from Poser 4. Poser 4 does include the Poser 3 models so if you upgrade, you won’t lose the ability to use the stuff you create for Poser 3 figures.
If someone has created a figure using their own original geometry and includes it with the cr2 file, you may be able to open it in Poser 3, but it won’t support the transparency maps if they are used in the figure. I believe Poser 3 requires textures in tif (PC) or pict (Mac) formats while Poser 4 accepts tifs, jpgs and psd files on the PC, not sure about the Mac.
Transparency maps, the grouping tool, support for importing and exporting more file types and conforming clothing are the items I remember being new to Poser 4 besides the new models. It’s been a long time since I used Poser 3 so I can’t recall all the changes.