jpegenc: Accept sof-marker=4

sof-marker is 4 when input is in the RGB colorspace.

https://bugzilla.gnome.org/show_bug.cgi?id=795463

ext/jpeg/gstjpegenc.c

diff --git a/ext/jpeg/gstjpegenc.c b/ext/jpeg/gstjpegenc.c
index 3f4be06..504f195 100644
--- a/ext/jpeg/gstjpegenc.c
+++ b/ext/jpeg/gstjpegenc.c
@@ -113,7 +113,7 @@
         "width = (int) [ 16, 65535 ], "
         "height = (int) [ 16, 65535 ], "
         "framerate = (fraction) [ 0/1, MAX ], "
-        "sof-marker = (int) { 0, 1, 2, 9 }")
+        "sof-marker = (int) { 0, 1, 2, 4, 9 }")
     );
 
 static void