matroskaenc: Bump DocTypeVersion to 4

The muxer has been creating files with v4 elements for some time now,
and especially now that we can mux non-experimental Opus files, reporting
the DocTypeVersion as 2 is not correct.

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>