In previous versions of Android Studio before 4.0, you can search across all text based files. Now, not only is the short cut to launch the search is gone. The "search everywhere" command only search across file names.
Did I miss something in the update.
You can still search across all files for specific text in AS 4+ by using:
ctrl+Shift+7
or from menu:
Edit > Find > Find in path
In case you don't know, the shortcut for 'Search everywhere' is achieved by quick double-press Shift. I have got AS 4.1. It works well, maybe something is missed when you updated, maybe the restart is required after the update. If restarting doesn't help, you can 'invalidate caches / restart'. Sometimes, IDEA/AS goes mad, fails to find references.