What is an MP4 file?
MP4 is a container, a box that holds video and audio streams plus metadata like chapters and subtitles. It became the universal video format because it can carry H.264, the codec every phone, browser, TV, and editor decodes in hardware, alongside AAC audio. Newer MP4 files may use HEVC (H.265) or AV1 video instead, which is where compatibility problems start.
Container versus codec
When an MP4 will not play, the container is rarely the problem; the codec inside is. A HEVC MP4 from an iPhone may refuse to open on an older Windows PC, and an AV1 MP4 will not play on older TVs. Converting to H.264 fixes it, and when the video is already H.264, tools like ours simply copy the stream into a fresh MP4 in seconds without re-encoding or quality loss.
Converting and compressing MP4
MOV, MKV, WebM, and AVI files convert to MP4 for maximum compatibility. Audio can be extracted to MP3. For sharing, the video compressor re-encodes at a lower bitrate and resolution, which typically cuts a phone recording by 80 to 90 percent. Short clips become GIFs for chat apps.
Frequently asked questions
What is the difference between MP4 and MOV?
Almost nothing technically; both derive from the same QuickTime structure. MOV is Apple's branding and is less widely accepted, so converting MOV to MP4 is usually just a container swap.
Why is my MP4 so large?
Phone cameras record at very high bitrates for editing headroom. Compressing to 720p at a medium quality keeps it watchable at a fraction of the size.
Does MP4 support subtitles?
Yes, as embedded text tracks, though support varies by player. Burned-in subtitles are more reliable.