I own lightroom 4 on one computer. I installed a lightroom 5 trial version on another computer for a trip I went on. I used lightroom 5 on the trip, keeping my catalog and all my pictures on an external hardrive. Now that my trial vertion for lightroom 5 is about up I need to use it's catalog in lightroom 4. How do I do it???
Short answer is: you can't. Catalogs are not backwards compatible, never have been with Lightroom.
Assuming you have processed those images in LR5 (if you haven't, just import them again into LR4), the best you can do is to select all images in the LR5 catalog and do Ctrl-S. This will write most of the metadata associated with those images back to the files (or to XMP sidecar files if using proprietary Raw format), though not all of it. Then you import the images into LR4 and hope for the best.....things like Keywords, Titles, Captions, GPS data should be preserved, but develop edits will be more of a lottery. Because the undelying Process Version is the same, some of the edits should be preserved, but anything done in LR5 using the new tools will be lost as LR4 will not understand them.
You cannot use a Lightroom 5 catalog in Lightroom 4.
The best you can do is to write the edits and other metadata to the files (select all, then Ctrl-S) using Lightroom 5, and then import the photos into Lightroom 4. If you used any develop features that are new in Lightroom 5, then those will not transfer to Lightroom 4. If you created collections, virtual copies, pick flags in LR5, those will not transfer. Edit history will not transfer (although final edited position will transfer)