The MP4 patent situation needs another close look. MP4, which was first standardized in 1998, ought to come out of patent soon, if it hasn't already. There are a few remaining patents in the MPEG-LA package, but they're mostly for stuff you don't need on the Internet, such as interlaced video, font loading, error tolerance for broadcast, and VRML. This hasn't been looked at hard since 2011[1] and it's time for a new…
If by MP4 you are referring to H.264, there are still many years remaining on most of the patents. MPEG-LA publishes patent lists, if you're interested to look. You are right in that there are many other encumbered technologies that have patents expiring soon. MPEG-1 and MPEG-2 video, MP3 and AC3 audio, and several container formats are included. Notably, this is almost all of the technologies required to make a DVD.
MPEG-LA has been padding their patent portfolio by dumping in all these patents on little-used features. Until this year, they still had some good patents, such as the ones on motion estimation, which is a hard problem and is needed to make compression work. But those have now expired. What's left looks like it can be avoided as unnecessary for Internet use.