If you already see MP4 thumbnails in Explorer then there's a quick way to get them in Opus as well:
Turn on Settings -> Preferences / Lister Display Modes / Thumbnails Mode: Use shell image extraction for formats Opus does not understand.
You'll then get thumbnails for almost everything that Explorer can generate thumbnails for, in addition to the things Opus knows how to thumbnail itself.
You can also enable Opus's Movie plugin to play and thumbnail MP4 files by installing an MP4 Splitter in addition to FFDShow. (If you're on a 64-bit OS then note that you need to install the 64-bit FFDShow and 64-bit splitters. You can install them in addition to the 32-bit versions if you want.)
More info about splitters etc. can be found in the FAQ on media playback.