The developers add some good stuff, but got rid of some excellent features. It's okay, I guess.....
Let's start with the general gameplay first. Along the game you'll play the game in two separate sides: the Durandal and the UCS. The good side is that each side gets different features as well as different wanzers. The downside is that you have to train both sides, which could be a hassle to do.
In battles, one excellent addition is the link system allows more than 3 more characters to attack a target simultaneously, very much like the Firing Squad ability in Front Mission 3, but now done more commonly. Another features are the radio, sensor and EMP backpack which endow your characters special features in battle. Another change is that now abilities are learned with AP points instead of learning an ability by chance, which can be frustrating at times... (however, this also reduces a bit of the excitement in gameplay as well)
One huge downside in gameplay is the loss of "internet," a feature appeared in the Front Mission 3 game. With loss of Internet, you now cannot catch criminals and get cash in return or at least download anything. This, therefore, simplified the game down to basic training and fighting game.
The story line is another huge downfall of the game, the plot is not as complex as Front Mission 3, so obviously not as thrilling to look at.
Well the music is not as good as Front Mission 3 either...
So, come to think of it..... I recommend you to get Front Mission 3 instead of 4. But 4 is still a decent game. Just be prepare for a bit of disappointment...