Is it possible to use Poser 6 figures in Poser 4?

Only to the extent that you can load them.  You'll get a warning about the Poser file version - you can safely ignore that.  But you'll be losing Shader materials (if any), morphs, and Face room support (head shaping morphs).

You can get the base morphs from the PMD (external binary morph file) inserted back into the CR2 file by using my Java application, MorphExchange (here in FreeStuff under "Poser:Utilities" or just search).

OK, I'll jiggle the files to see if I can load. Thank you very much..

Hmmm, I transfer all data sometimes to poser pro 4 and loose nothing.  Sharen

So I see that I'm not the ONLY one who doesn't see Poser 6 as the all encompassing 3D Savior that some have made it out to be.  As far as I'm concerned, Poser's last decent update ended with Pro-Pack.
David P. Hoadley

SAMS3D, really depends on how you do it.  If you try to directly load an unaltered Poser 6 figure from the library there is no possible way that you can have access to the morphs stored in the PMD file (as anything before Poser 6 has no idea what that is) and no access to Face Room morphs (not even in Poser 5 as these are in a new format).  Shader nodes will have no problems in Poser 5, but will be completely lost before that.

Now, if you save the figure back to the library or in a PZ3 with "Use external binary morphs" disabled and "Use file compression" disabled, you will have access to the morphs and the file in Poser 5 and earlier.

Interesting -- but where's the option to "use external binary modes?" Could you please spell out the steps? 

Go to the Edit menu
Select "General Preferences"
Go to the "Misc." tab on the window that opens.
Under "Save Files" there is a checkbox for "Use external binary morph targets".

• Unchecking will cause any scene saves or additions to library to have regular old morphs.
• Checking will cause the same to have a .PMD file created which contains the morphs in a binary format.

Quote - So I see that I'm not the ONLY one who doesn't see Poser 6 as the all encompassing 3D Savior that some have made it out to be.  As far as I'm concerned, Poser's last decent update ended with Pro-Pack.
David P. Hoadley

Why, because you can't import newer Posr 6 features back into an obsolete version of Poser?
Sorry, but Poser 6 is head and shoulders above the previous versions.

Wow -- thank you very much!!

There is one other thing that should be mentioned - Since Poser 5, all included Poser content files are zlib compressed.  Since Poser 6, this also includes the Wavefront OBJ geometry (with file extension .obz).  Poser 5 cannot load the compressed Wavefront geometry files and versions prior to 5 cannot load any compressed files.

There is a Python script in P5 and P6 that can decompress files.  You can also disable file compression in both versions.  You can even use WinZip or just about any other archive application that supports zlib compression-decompression to convert these back to regular text files.

Be aware of this before attempting to load Poser 6 content with Poser 5 or earler.

Robert

"Why, because you can't import newer Posr 6 features back into an obsolete version of Poser?
Sorry, but Poser 6 is head and shoulders above the previous versions."
NO!  THAT IS NOT THE REASON!  IT'S BECAUSE POSER 6 IS SLOWER THAN FROZEN MOLASSES IN AN ANTARCTIC WINTER!
More complicated should NOT be confused with better!
David P. Hoadley

I never saw PP so I can't comment on that, but the appalling materials editor in Poser 4 really hamstrung the program. Plus the fact that the library system was designed without any idea of how it would cope with thousands of files. Improving those two features really elevated Poser 5. Poser 6 as far as I'm concerned is mostly a bug-fix for Poser 5. It's a bit slicker.