Cafe Moto Vespa Club has a great review of the MP4/MP3 400 up:
The first thing I noticed was the weight of the bike. It's a full 212 lbs. heavier than my GTS, so when rolling to a stop, you can feel the additional weight of the bike. Like every big displacement scooter I have ridden, there is a small vibration when starting out from a stop which goes away at about 15 mph, and from there the acceleration is smooth and the power available with just a slight flick of the wrist. The MP3 400 doesn't take off from a dead stop as quickly or smoothly as you would on a GTS, largely because of the weight, but there is plenty of power all through the power band.
Riding on the freeway I really understood the difference between the MP3 400 and my GTS. The MP3 is smooth as silk at freeway speeds, and there is power to spare. Here is where the extra 212 lbs. turns into a real asset, as you glide along without feeling buffeted by the winds. I can see riding this bike on long trips in total comfort.
Comments on the bike itself: The seat is comfortable, but then I like riding more forward than some people do. I like the fact that the low of your back makes a positive stop against the upper level of the seat, which gives you a little back support. I think that little positive stop in the seat also gives more control of the bike. The windscreen on the other hand is relatively useless. This bike was made for higher speeds, and it needs a proper windscreen to cut down on the wind and noise. ...
If you spend most of your riding time in town and in traffic, the 400 might be a little frustrated. But if you want a bigger bike with power for highway travel, and agility in the twisties, this may be the bike for you. Think of the MP3 400 and 500 as the scooter equivalent of a Sport Bike, and you'll have the idea.
Click the title link to read the whole review.
As far as I know we won't be getting the MP4 at Vespa Ridgefield, with the mere $200 price difference between it and the Fuoco/MP5/MP3 500, the owner figures that most folks here would just opt for the extra 100cc's.