You can always launch LR 3.6 which will use the LR 3.6 catalog as you last saw it. There is no way to move images in LR4 back to LR 3.6, you would have to re-import these image into the LR3.6 catalog, probably losing your edits and metadata.
You can select all the new imports in LR4 and Ctrl+S to write the metadata back to the files, then when you import to LR3 you'll get some of that metadata brought in, e.g. any keywords and ratings and colour labels.....but as Paige says you won't get any develop edits (LR3 won't recognise them), they'll be imported with your LR3 default develop settings.
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