For single file you can use FourCC changer. My tool is for batch processing.
FixAvi4cc.exe [options] dirName1 dirName2 fileName1 fileName2...
FixAvi4cc.exe -backup -skipReadOnly -skipCheck -topDirectoryOnly -log logFileName dirName1 dirName2 fileName1 fileName2...
-backup Backup original file
-skipReadOnly Skip read-only files
-skipCheck Skip FourCC checking equals 'xvid' or 'divx'
-topDirectoryOnly Search avi files in top directory only, not recursive in subdirectories
-log logFileName Log output to logFileName
-noQuestion Skip asking for continue
dirName1 dirName2... Paths to directories where to search .avi files
fileName1 fileName2... Paths to .avi files
-?,-h,-help This help output
all arguments are not required, by default it searches all .avi files in current directory
and subdirectories with xvid/divx FourCC and changes it to FMP4 without backup(read-only files too)
Based on this thread