Sorry for responding so late, jusT seen your post.
I have a 2009 MKX and the latest (and last) sync update is 2.1.4. I have navigation, Sirus & DVD. (Sync version's vary depending on the equipment, so 1.3 may be the highest you can install; just don't know).
No, you cannot go to version 4, it's a hardware limitation, not software.
Imo, even if you installed all of the upgraded hardware, to go to Myfordsync, you wouldn't gain much, and it would be expensive.