Add `ac4` to known codecs

Note: ffmpeg does not currently support this format

Related #5738

README.md

@@ -1488,7 +1488,7 @@ The available fields are:
  - `source`: The preference of the source
  - `proto`: Protocol used for download (`https`/`ftps` > `http`/`ftp` > `m3u8_native`/`m3u8` > `http_dash_segments`> `websocket_frag` > `mms`/`rtsp` > `f4f`/`f4m`)
  - `vcodec`: Video Codec (`av01` > `vp9.2` > `vp9` > `h265` > `h264` > `vp8` > `h263` > `theora` > other)
- - `acodec`: Audio Codec (`flac`/`alac` > `wav`/`aiff` > `opus` > `vorbis` > `aac` > `mp4a` > `mp3` > `eac3` > `ac3` > `dts` > other)
+ - `acodec`: Audio Codec (`flac`/`alac` > `wav`/`aiff` > `opus` > `vorbis` > `aac` > `mp4a` > `mp3` `ac4` > > `eac3` > `ac3` > `dts` > other)
  - `codec`: Equivalent to `vcodec,acodec`
  - `vext`: Video Extension (`mp4` > `mov` > `webm` > `flv` > other). If `--prefer-free-formats` is used, `webm` is preferred.
  - `aext`: Audio Extension (`m4a` > `aac` > `mp3` > `ogg` > `opus` > `webm` > other). If `--prefer-free-formats` is used, the order changes to `ogg` > `opus` > `webm` > `mp3` > `m4a` > `aac`

yt_dlp/utils.py

@@ -3572,7 +3572,7 @@ def parse_codecs(codecs_str):
                 hdr = 'HDR10'
             elif parts[:2] == ['vp9', '2']:
                 hdr = 'HDR10'
-        elif parts[0] in ('flac', 'mp4a', 'opus', 'vorbis', 'mp3', 'aac',
+        elif parts[0] in ('flac', 'mp4a', 'opus', 'vorbis', 'mp3', 'aac', 'ac-4',
                           'ac-3', 'ec-3', 'eac3', 'dtsc', 'dtse', 'dtsh', 'dtsl'):
             acodec = acodec or full_codec
         elif parts[0] in ('stpp', 'wvtt'):
@@ -3605,7 +3605,7 @@ def get_compatible_ext(*, vcodecs, acodecs, vexts, aexts, preferences=None):
     # TODO: All codecs supported by parse_codecs isn't handled here
     COMPATIBLE_CODECS = {
         'mp4': {
-            'av1', 'hevc', 'avc1', 'mp4a',  # fourcc (m3u8, mpd)
+            'av1', 'hevc', 'avc1', 'mp4a', 'ac-4',  # fourcc (m3u8, mpd)
             'h264', 'aacl', 'ec-3',  # Set in ISM
         },
         'webm': {
@@ -6048,7 +6048,7 @@ class FormatSorter:
         'vcodec': {'type': 'ordered', 'regex': True,
                    'order': ['av0?1', 'vp0?9.2', 'vp0?9', '[hx]265|he?vc?', '[hx]264|avc', 'vp0?8', 'mp4v|h263', 'theora', '', None, 'none']},
         'acodec': {'type': 'ordered', 'regex': True,
-                   'order': ['[af]lac', 'wav|aiff', 'opus', 'vorbis|ogg', 'aac', 'mp?4a?', 'mp3', 'e-?a?c-?3', 'ac-?3', 'dts', '', None, 'none']},
+                   'order': ['[af]lac', 'wav|aiff', 'opus', 'vorbis|ogg', 'aac', 'mp?4a?', 'mp3', 'ac-?4', 'e-?a?c-?3', 'ac-?3', 'dts', '', None, 'none']},
         'hdr': {'type': 'ordered', 'regex': True, 'field': 'dynamic_range',
                 'order': ['dv', '(hdr)?12', r'(hdr)?10\+', '(hdr)?10', 'hlg', '', 'sdr', None]},
         'proto': {'type': 'ordered', 'regex': True, 'field': 'protocol',