Mega Man 4 is not a bad game, despite how often I may seem to be calling it such. The main stages flow nicely, the end game stages keep their quality up much longer than previous games, and there were a lot of great new stage mechanics thrown in.
We got a full water stage, plenty of new platforms to mess with, hidden items, split paths, and a variety of environmental effects. Some of these worked out great; others, not so much.
The problem is that the longer you stop to think about it, the more disappointing the failures become. Dust Man's stage had huge pistons moving around, but they only lasted for a few screens and only one was at all threatening. Bright Man added two types of platforms that move in an arc. Unfortunately, the player never had much reason to move backward with the green version and their movement wasn't used to do things that horizontally-moving platforms couldn't. The mechanics behind Drill Man's falling rocks were awkward and the switches were pretty weak as a stage gimmick. Ring Man was almost fantastic, but the inclusion of four annoying mini-bosses made it infamous instead. Cossack introduced timed spike platforms that didn't make use of their staggered timing and were never seen again. And so on.
While none of these get in the way of enjoying the game directly, their cumulative effect causes greater harm than a few individual bad portions would, as the game lacks the highs and lows that a player can latch onto in memory. Mega Man 2 had some nasty bosses, Quick Man's lasers, and the dragon. Mega Man 3 had, well, all of this:
Mega Man 4 got a lot of the small stuff right (Taketento is far more interesting than many previous flying enemies, for instance), but it lacked these big moments; the platforming difficulty spikes and the dangerous enemies. The hippo is probably the first enemy to spring to mind, and it's remembered mostly for wasting the player's time.
The worst aspect of this game is the boss fights. Mega Man 3 had two easy fights (Magnet Man, Top Man), two that took a moment to figure out but weren't that hard (Snake Man, Spark Man), and four fairly brutal bosses (Hard Man, Gemini Man, Needle Man, Shadow Man) with the last taking things a little too far.
Mega Man 4, on the other hand, has two pointlessly trivial fights (Toad Man, Dust Man), and two fairly good ones (Pharaoh Man, Ring Man). The rest were mildly difficult but terribly boring to fight, with Drill Man spending too much time underground and the rest relying more on contact damage than neat attack patterns.
The main bosses are the true stars of any Mega Man game, and 3/4ths of them failed to impress this time around. Even Wily himself was a serious let-down, reverting from the walking robot to the old type of flying machine spitting simple projectiles. The Wily Capsule was a fine idea, but it was also too simple in this form.
The weapons are a tossup. They're more useful than Mega Man 3's (especially since the Flash Stopper and Rain Flush can trivialize almost anything), but are also more boring. I had little reason to use the Gemini Laser, and the Spark Shock was often detrimental to Mega Man's health, but they at least offered ways to attack enemies that the Buster doesn't. Most of 4's weapons fire forward or hit the whole screen, with the Pharaoh Shot being the only one to stand out as both helpful and fun. I can also give the Skull Barrier credit for its usefulness despite how weak it looks at first.
On the other hand, I seem to be one of the few who doesn't consider the charge shot a bad idea (awful noise aside). The only time it felt forced was when fighting Whopper, and it's an excellent crutch for players who can't or don't wish to shoot as fast as the game sometimes wants them to. It does make boss fights easier, but I blame the design of those bosses more than the shot, as more agile enemies would make it riskier to use. The other items were also handled better here, as Rush Jet now asks a little thought from the player when using it, Rush Marine gets a full water stage, and the optional items were surprisingly useful throughout the end stages.
Though there's no getting around the drop in quality of the main boss fights, the rest of the game is still absolutely worth playing. It looks nice, has plenty of good moments to balance the bad, and it threw enough new ideas into the mix that its failure to follow through on many of them can be forgiven if you just sit back and enjoy the ride.