I've had the Musha 3, 4 and now the 5. The 5 is the lightest. That however doesn't necessarily mean its the best.
I thought the 3 was ok as a daily trainer. I got it mostly to have a lower Heal-To-Toe ratio that Mizuno's standard 12mm offset.
When the 4 came out I immediately got them and wow! The 4 is a really nice shoe.
The upper is just fantastic, it is like a spike since it has that "dancing pump" shape in the forefoot.
Now the 5, has moved from the dancing pump feel to more of a slipper, similar in construction to the Ronin, which I also have.
It is a little bit tighter in fit than the 4 or 3, but it is true to size.
Over all this is a really nice shoe but would I buy it over the Musha 4, no. But that is just me. If I could give it 4.5 stars I would.