It sounds as though you've opened a different catalog on the desktop, i.e. not the catalog that you originally synced those 4 collections from? Is that correct? Only one catalog can be in sync with LRmobile at a time, and if you open a different catalog and try to enable syncing then you'll get the warning message that all existing synced data will be deleted.
Is that what happened, and did you confirm the "all data deletion" warning message?
Thank you, Jim. I only have one catalog though and I sync'd these 4 collections from that same catalog. I've been having some odd issues with the USB external HD I use to store pictures, related to how Yosemite handles hard drives going to sleep. Lightroom throws all kinds of catalog read errors/backup errors after the computer has slept and I haven't unmounted/remounted the drive.
I have not confirmed the "delete all data" yet, I want to be sure it won't delete the work i've done.
Rethinking the issue, I wonder if I could fool LR into thinking I uploaded the collections from my iPad? So it wouldn't remove all the work? But then, would the mobile-size files relink the changes I've made to the full resolution versions on my desktop?
I really like the promise of the LR mobile workflow, but sync errors like this break the whole thing. Couldn't LR look and see which metadata is most recent and reference that?
God help me if this were a client project, it's only my wedding and honeymoon photos.
OK, in which case I'm not sure what's happened here. Can you post a screenshot of the exact message you get when you open the catalog on the desktop?