Impossibly difficult after 3 completes, but very addicting.
Furthermore, the fact that you only have 3 turns for a "style change" really irritates me. MBN 3 was perhaps the best game in the series. At least I had great skills throughout, and the viruses were truly leveled depending on where I was. The Mettaur in ACDC 2 stayed Mettaur, no matter how many times I deleted Alpha Omega. In 4, I feel as though I'm going through the same area throughout the game. The only thing that changes is the way the attacks are delivered.
As frustrating as this game may seem, I still find myself going back to it. It may be how great it feels when I actually defeat Duo at his hardest. His attacks cover 2/3 of the battlefield (sometimes all of it), and I actually have to aim my attacks instead of just throwing out my big chips, which I usually use when quick moving viruses are all over the field.
I wouldn't suggest this game to someone that isn't skilled at Netbattling from the start of the game. I had great practice with 3, both with the Secret Area and friends that had the game. Without that practice, I would probably have stopped playing before I reached the Red Sun/ Blue Moon Tournament.