With this version there are a set of breaking changes that must be considered when upgrading from a previous version.
Default image
Ind the previous version, if no valid quality was set or available, the fallback was the source of the image. In this version the fallback is configuable DFO 4.0.0 - 3 Setting up Optimizely
Look for the property MediaElements. The default mediaFormatId is 2. If you want the source, then you can append /source to the url. like this globalassets/digizuite/997345-en-butterfly-1-1535829.jpg/source.
Find indexing
Ind the previous version assets was automatically indexed. This is no longer the case, assets wont be index out of the box. Follow this guide to ass addets to the index DFO 4.0.0 - 8 Indexing Assets with Optimizely Find.