Find all files that match a filename pattern in a folder tree
As you tagged this Python 3.x, I'd suggest using pathlib.Path.rglob() and collections.defaultdict.  Also, check out f-strings.

import collections
import pathlib

mp4s = collections.defaultdict(list)

root = pathlib.Path(r'/mnt/mp4library/mp4library')

file_count = 0

for filepath in root.rglob('*.mp4'):
    #filepath is relative to root. Uncomment the next line for absolute paths
    #filepath = filepath.resolve()

    mp4s[filepath.parent].append(filepath.name)

    file_count += 1

for record_number, (folder, filenames) in enumerate(sorted(mp4s.items())):

    print(f"-----record {record_number}---{folder} ... files={len(filenames)}")

    for c,filename in enumerate(filenames):
        print(f"mp4 File {c}---filename")

print('Count of files: {filecount}')