I disagree with anyone who says 4 is better than 5.
5 has everything good about 4 with none or less of the bad. 4 refreshed the series by introducing three new characters, but was effectively worse than 3 in many ways. An example of that was how wimpy the bosses were. 5 fixes this quite a bit (and also provides the second best final boss in the series, if not the best in terms of pure spectacle). The general enemy AI was also improved by some extent (and they added reinforcements), but it unfortunately basically begs you to equip the Champion's Ring (and make sure you leave in storage before moving onto the next chapter/character), which makes enemies more aggressive/swarming. I can't say for certain, but I feel like AI has gone downhill in the series since maybe 3 (if not PS2 era?) and Champion's Ring is their band-aid for that. While you can still easily overpower the game with equipment, the floor has been raised due to enemies being being better and more interesting overall.
As far as pacing goes, the only real slump is Saejima's chapter. He's the least interesting character to play and his chapter is weirdly structured (may be worth skipping his quests if they begin to become boring). On the other hand, you can say 5 has five parts (including the finale), whereas 4 suddenly rushes the ending, leaving you with three and half parts plus a series of (disappointing) boss battles. Really, I can't get over how solid (and hype) 5's finale is, whereas 4 only as one really badass intro cutscene to show for itself.
What's particularly noteworthy is how goddamn big it is. It features five cities, with four extensive mini-game systems that switch up the genre completely (taxi/racing, idol sim, hunting, baseball batter). Despite its size, it's pound for pound as or more polished than any of the previous games. It has a new engine over 3/4 that looks much better. Plot wise, considering the kind of fuckery 2, 3, and 4 got down to at times, it's surprisingly more grounded and/or coherent than usual (which, granted, is still pretty crazy), despite its scale and complexity. The soundtrack is probably a little better than 4's, but not as good as 3's; the only issue is that it doesn't truly blossom until the finale (or the last chapter of Part 4), when you get a whole other set of a unique boss themes thrown at you.
I don't think it is a stretch to say it's the best game in the series. (This whole post is in reference to the western released games only, although I didn't play Dead Souls, for what that matters.)
So yeah, it's worth a buy, especially if you don't plan on playing any of the previous games first.