This game is still a MMZ game. So it's awesome. That being said though, after replaying them all recently, this is the one I find myself thinking positively of the least. It's hard for me to really pinpoint anything specific; but I think my main issue with the game is simply the level design. Many feel very... uninspired. Notably, the only good level out of the 4 stages you get before the final ones I'd say is Cubit Foxtar. While the other games have their stinkers too, they aren't as bad as they are here I'd say. I also find the bosses here to be probably the least interesting in the series. The part system I find pretty lame because most of the time they're either useless or busted, so I end up not using anything. This game is definitely still good (like I said, MMZ) but I don't think it's the best in the series anymore.

Original Review
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The culmination of the MMZ series. While 4 is technically the last, that game set itself apart from this one with new things specifically because it knew this was already the perfection of the formula. This game completely cuts away the weapon grinding from the past 2, adds a kickass new weapon (the recoil rod), and has more stages than the past ones. The only thing I think you could actually argue is a regression is the forms from mmz2 being replaced with chips, which I find to be less interesting.
This has all the reasons the first two are good, the fluid and rewarding combat, the tight controls, the grim setting, the good music, the ranking system, topped off with an actually good story for a GBA platformer (though admittedly mmz1 probably has a better one).
The improvements from the previous ones aren't super substantial, but all that means is the mmz series always had what it took to be great. It really is just improved level design and greatly reduced grinding that elevates this one above the others. My only real issues is that this game can be really easy if you exploit certain things (element chips, cyberspace, cyber elves), but previous games had this too and you can ignore these, just thought I should mention it. Also the fox boss is weirdly hard compared to all the other ones for some reason.
The worst part of this game is that I can't recommend this game by itself as the story requires buildup from 1 and especially 2 to be worthwhile, with the story actually being important enough to want to have the best of, but that's hardly a fault. This game rules.

... ... ... Megaman Zero 4... to live or die...? Hmm ..... ...

This is probably the MMZ game I have the most conflicted feelings over. 4 tries to set itself apart from previous entries a lot, and most of the time I just don't think it works out well.

The parts system is complicated enough to require a guide, the EX skills now have their elements tied directly to them which takes out a lot of potential application, the Elf system got completely reworked, and the Shield and Rod both got removed in place of... the pretty useless Z Knuckle. One new system I do think is cool though is the weather system, adding effectively a "hard mode" to the levels, which, even if it's not much, is appreciated for the bit of extra challenge. While I can respect the desire to stand out, I think this aspect of Zero 4 doesn't do it too many favors overall.

The level design of 4 is good, I'd say. It's better than 3's probably and more consistent than 1, but some levels I find to be pretty bland (Deep Sea, Hibernation Chamber, second half of Magnetic Field) though they're still enjoyable because you're playing as Zero of course it's fun. Bosses in Z4 are very not consistent however, sometimes they're the best the series has to offer like Fenri Lunaedge or Sol Titantion, and then other times you're fighting Mino Magnus or Heat Gemblem.

I don't really know how to wrap things up here, Zero 4 is still a Zero game so it's great but I just don't enjoy it as much as I do some of the previous games.