I had downloaded a trial of Lightroom 4. Did some edits from one of the shoots. Now the trial has expired (I can't buy it yet, as budget won't allow me) I'm not able to see these changes in my Lightroom 3. I tried synchronising, but that didn't help. Any advice how to get or where to find what I've already done? Thank you!
When you installed LR4, you would have upgraded your catalog for it. The naming of these are based on the versions, eg:
Lightroom 4 Catalog.lrcat and so changes that are contained in that catalog will not be visible in the catalog for LR3.
Now, the way you can make sure that losing edits doesn't happen in the future is to ensure the changes are written to sidecar XMP files. That's part of the "Catalog Settings" on the Metadata tab (I don't have LR3, just 4 and 5, but I assume such an option is available there as well). You want to do this, regardless, because it's defence against crashes, corrupted catalogs, and the like. Put it a different way: it will only help you. See this tip for details.
In the meanwhile, I'd suggest seeing if you can use the LR5 trial to get some XMP data written out. It might not work, it depends on how clever Adobe is with trial versions. The upgrade, by the way, for this version is very reasonably priced.