Keep in mind this is coming from someone who thinks FM3 is the best game on the PlayStation, but I thought Front Mission 4 was definitely worth playing. The whole point buying system was brilliant and single handedly erases any memory of the demo (which seemed very shallow thanks to the buying system being disabled).
Battles are not Ring of Red long (unless you're reviewing the game for IGN) but they tend to be longer than FM3 levels. The AI is basically the same as in 3, which irritates some people but also tends to be pretty important to how the game plays.
Diomedeskun's comment about bodies actually just depends on how you set up your team. I typically keep my short-mid range brawlers with high evade gear, because once you start chaining and boosting, prepare to dodge 5+ attacks in a row. I tend to keep long range (Bazooka/Missile) units on high HP armors, with Supply/Sniper types on balanced wanzers (since they seem to do best with quad legs or hover legs).
The customization really is the strongest in the series; you'll actually WANT to play training just to get more money for stat upgrades. Some of the Wanzers can end up with 35+AP, which makes for some insane link strategies (necessary to get through the secret training levels).
I still think FM3 is a stronger game (better story, 2x as long, actual humans, and so on) but FM4 was a real breath of fresh air for the PS2 lineup. The game also has a new game+ mode, essentially liquifying all of your purchases and cash and then giving it back to you at the beginning of the next playthrough in the form of a lump sum.
If you want a more balanced look at the game (including the points that inevitably drag the game down from being an all-time classic), I talk about it in my FM4 war journals
here.