Fuzzy Chickens wrote:But how many PROGRAMS are capable of importing and editing .mp4 files?
Doesn't matter. Don't edit with distribution encodes, they aren't and never were meant to be edited with. Support of it via a proxy like AviSynth is the only safe way to handle such files short of a full reconversion to the lossless format of your choice (HuffYUV, ffvhuff/HuffYV12, Lagarith, FFV1, H.264 Lossless, Lossless JPEG, CorePNG, Uncompressed, etc.).
Unless by 'importing' and 'editing' you mean purely container (ex. concatenation, splitting, or mux/demux) and metadata-related tasks. MP4Box and apps like AtomicParsley can do that just fine.