Save/export merged video as H.264 MP4 video file.
After Media Composer project editing is complete:
Place blue scroll column at first frame of sequence
Add MARK IN
Place blue scroll column at last frame of sequence
Add MARK OUT
From the menu at top:
File -> Output -> Export to File
Export setting = Send to QT Movie
Export As: MP4
Check "Use Marks"
Check "Use Selected Tracks"
Compression = H.264
Audio Mix = Stereo
Click Save again to generate the file.