Version: Rhino 4
The SaveAs feature in Rhino 4.0 can be used to write files that Rhino 3.0 can use. By default, plug-in user data is not saved when you save a V3 file from Rhino 4.0. If you feel need this, and you are willing to do some extra coding and lots of detailed testing, it is possible to round-trip plug-in user data between Rhino 3.0 and Rhino 4.0.
There are several approaches available. The one you choose depends on what you want to accomplish.
There is not a V3 version of the plug-in. You want your V4 plug-in to be able to round-trip information through V3 files.
There is a V3 version of the plug-in. You want your V4 plug-in to be able to round-trip information through V3 files. You do not want the V3 and V4 plug-ins to exchange information.
There are V3 and V4 versions of your plug-in. You want the V3 and V4 plug-ins to share information.
This level of support requires expert programming, through testing, and thoughtful file IO code design. In particular, if you have changed the information your plug-in saves and you did not carefully plan for this situation when you wrote your V3 plug-in's IO code, then it is likely you cannot provide this level of file support for your users. To properly test your code, you must be able to run your V3 and V4 plug-ins in a debugger.