Simon -
I bought the MD4 when it first came out, and after using a Tascam 4 track for years it was quite a difference. Much cleaner, no hiss, etc. BUT - the MD4 uses Atrac compression which throws away some data to reduce file size, and it is AUDIBLE, especially after you bounce tracks. Also, the MD4's pres are lousy, you will need a good mic pre with it.
The Roland is not compressed, and gives you "virtual tracks", which the Yamaha doesn't. Alaos, those ZIP disks can be easily downloaded to a computer, whereas the Minidisc has to be recorded via analog I/O. If I were faced with your choice, I would get the Roland. Good luck!
Bob