I can initially think of two things: corrupted or only partially downloaded image from juniper.net or lack of disk space on the MX204.
Can you verify that your image for 19.4R3 matches the following sha256 hash via 'file checksum sha-256 /path/to/file.tgz'
21987f99b408c442e1222caaa182bcbd06d46dfba341852283ad0bfd4b89be36
Secondly check that you are not lacking disk space via 'show system storage' - and if in doubt do a 'request system storage cleanup dry-run' to see if any large files shows up in the list. You can then either manually remove specific files or just remove 'dry-run' from the command to delete all files listed via the previous command.
Let us know how you progress with the upgrade with the above inputs.