Thanks, Elliot-
The link above wasn’t as clear as I was expecting, but with the help of others on the forum I was led to downloading the plugins, and while I did still lose my database completely I was able to get going again.
For those in a similar situation: it’s actually a lot better if you find yourself accidentally upgraded to v.4 to NOT try and downgrade. This is possibly what killed my site and custom fields. Elliot is right: on the surface, not much has changed, and most of old code is still supported in v.4 (the use of the repeater field in templates, for example).
If I had taken a breath and found that the only thing necessary to make the new version work is install the add-ons as separate plugins, I would have probably been fine. Don’t do what I did!
