Ah, another remark, might be important depending on what addons you use:
In P3Dv4, you have to forget about anything from FS9. FS9 addons will not work at all, they won't even display. Also, for FSX addons, any gauges (.gau or .dll files) in 32 bits will not work in P3Dv4. XML gauges will be fine, though
In P3Dv3 (at least the version 3.3, I don't know if the statements below are still true with version 3.4...), which is still 32 bits like FSX, FS9 aircrafts will mostly work like in FSX, with the following similarities or differences:
- 2D gauges in virtual cockpit will not be clickable anymore. So, it you have a FS9 plane with a VC with 2D buttons, these won't be clickable anymore.
- However, any button or lever modeled in 3D will still work and be clickable

- You will still get the transparency problems for props or canopies, but this can be adjusted by editing the textures, same as in FSX.
- you will NOT get any "props/windows disappear behind the clouds/autogen" problems, contrary to FSX
- you WILL get self shadows in the exterior model
- you WILL get shadows in the virtual cockpit !
That being said, the nicest freeware planes from FS9 are constantly being converted to FSX-native models with XML gauges, which means they become fully usable (even when they have 2D buttons) in P3Dv3 or v4, so you might want to search for the latest version of a plane before taking any decision.