Mediocre in every way.
Story and characters? They suck, and for an RPG that's pretty bad. Not once was there any kind of tension or emotion brought up by what little story this game has. The characters are pitifully one-dimensional, and you won't give a single fuck about anything that happens to them, or anything that happens around you for that matter (server is under attack by the X Virus? Oh noooooooo, how awful. Fucking whatever, man). The only characters you might care about even slightly are the female Digi-Elves, who are actually kinda cute-looking and fill your cold heart with warm, tender feelings, but that's not enough to save this game.
Gameplay? Boring and tedious. After the training level, you're thrown into the first world, where you WILL get your ass kicked, forcing you to constantly warp back to the main hub and re-enter the first world time and time again until you level up enough to afford making mistakes. Controls aren't actually too bad, to be fair; guarding takes a while to get used to, since you have to block just as you're about to be hit, but it works. Unfortunately, this game includes collision damage, which not only doesn't make sense from a logical perspective (last I checked, skin-to-skin contact doesn't cause spontaneous bleeding), it makes combat frustrating until you're strong enough to one- or two-shot enemies, because most of the time you'll be up close and personal to kill them; none of the starter Digimon are proficient with guns, and MP for projectile attacks drains very quickly at the beginning of the game. And don't even get me started on levelling up your techniques, which take fucking forever, especially for MP techs. Ultimately, the combat quickly becomes unfulfilling; there isn't a real goal to aspire to, and you just feel like you're grinding for the sake of grinding. When playing a game feels like a chore, something's wrong.
One other thing that bugged me was how you could replace all the Digimon models with something completely unrelated to Digimon, slapped a piece of paper saying 'An Action RPG' on the game cover, and it wouldn't make any difference to anything in this game. It's Digimon In Name Only
Is Digimon World 4 good? No. Will you get bored of it after a few hours, unplug your machine and walk away disappointed? Yes.
Your mileage may vary, though. Play it yourself and make your own damn opinion.