How to convert NTFS to ext4 in windows

The best idea I can think of is to boot to a Linux LiveCD that supports ntfs read/write and mount the ext3 partition. Then hookup an external drive, format it ntfs / fat32 (as long as you don't have 2GB+ files), then copy all the files over. Then boot back to windows, hookup the drive, and you'd be able to access the files.

Another option is to use the same process, but copy to the windows partition instead. The main drawback is that if something goes wrong it could destroy your windows partition, hence why an external drive is recommended.

What is 6G mainly used for, for journal, for redundancy, or for indexing?

(Not known yet.)

Why the remaining space 393G not 415G? There is a 22G hole which is rather big.

It's 5% reserved blocks for superuser, which is used to avoid fragmentation. You can adjust it to 1% by mkfs.ext4 -m 1.

What parameters would you advice if this ext4 partition is used to store movie/music files?

You can specify a usage option to mkfs.ext4, e.g., mkfs.ext4 -T large_file, this will let mkfs.ext4 to decide parameters for partitions containing large files.