Hi - I'm just trying to get something looked at by ESRI support :-s and it would be fantastic to get some other confirmation/denials on the issue I've found. It seems that any AddIn which contains folders (i.e. there is some kind of folder structure within the Install area of the add in file) will fail to load up in ArcMap 10.4. It's always been fine in previous versions, and I even noticed that the standard MapCenter AddIn hasn't got folders in since version 10.4 - and always has had up to 10.3.
Attached is an extremely minimal (68kb) AddIn/procedure to show the problem. If anyone developing AddIns and using/moving to ArcMap 10.4 could just try this out on your own AddIn for some extra confirmation I'd be extremely grateful. Just add a folder inside the "Install" bit of your AddIn (can be empty) and see if it fails after the next upgrade....
This happens with both Java and .NET addins.