Sprockets 4 has a new way of handling which files to compile, which requires a bit of setup.
Add a file app/assets/config/manifest.js with this content:
//= link_tree ../images
//= link application.css
//= link application.js
This will work with a standard setup, but you might need more links if you have a differen setup.
You can now remove any additions to config.assets.precompile you may have.
More information can be found here: https://github.com/rails/sprockets/blob/master/UPGRADING.md