Mega Man IV is the sequel to the Game Boy game Mega Man III. The Roman numerals are meant to distinguish this series from the NES games on which they are based.
Mega Man IV follows its predecessors in taking four "robot masters" each from two NES games, in this case Bright Man, Pharaoh Man, Ring Man, and Toad Man from 4 and Stone Man, Crystal Man, Charge Man, and Napalm Man from 5. The stages are remixed both so as to provide something new and because the Game Boy's small screen made it impossible to reproduce them exactly.
As in all Mega Man games, when he beats a robot master, he gets his weapon. My main gripe with Mega Man games in general is the limited usefulness of most of these weapons (except in Mega Man 2, where the Metal Blade is superior). The default Mega Buster, which can also be charged up for a more powerful attack, is so versatile that it's rarely worthwhile to use another, except to exploit a robot master's weakness. Each master is weak to one weapon, or sometimes two. I did occasionally use the Pharaoh Shot, because it can also be charged and shot at a diagonal, which the Mega Buster can't. Also, the Ring Boomerang can pick up items. These are sometimes placed behind walls, so that the Ring Boomerang is the only way to get them.
Mega Man controls very well, the same as in the other games. He can jump and slide but not run. He skids a bit when stopping, which can sometimes cause him to fall off an edge if you aren't careful. I don't mind this because it's realistic and adds a little challenge without being frustrating. Like many 8-bit games, there is lag when too many sprites appear on screen at once. That's just the way it was back then.
The biggest difference between this game and its predecessor is that this one is easier. Mega Man III, as great as it is, is on the more difficult side. This game feels like a deliberate attempt to correct that, which I appreciate.
Not only are the levels easier, but this game also introduces something new: a store! In addition to collecting health and weapon refills, enemies now drop "P-Chips." This form of currency can be spent in Dr. Light's lab between stages. It's not apparent, but you can also access the lab from the stage select screen by pressing SELECT.
The first thing I bought is the new Auto-Charger. With this, weapon refill capsules are automatically applied to whichever weapon has the least energy. No more opening the menu to switch back and forth between weapons to refill them! Energy tanks (E-Tanks) are the main thing you'll want to get (they can also still be found in stages). Two other types of tanks are introduced as well: the S-Tank, at twice the cost of an energy tank, fully refill everything, including Mega Man's health. Mega Man can only hold one at a time. The W-Tank, at half the cost of an energy tank, fully refills one weapon. There are also tiny E-Tanks: when four are bought and/or found, they become one full E-Tank, like heart pieces in the Legend of Zelda. You can also buy extra lives. With all these items, the game is both more fun and easier.
Like in the Blue Bomber's other Game Boy outings, after defeating the first four robot masters, there is an extra stage with a "Mega Man Killer" robot not taken from the NES games. This time Mega Man must face Ballade, from whom he obtains the Ballade Cracker weapon. A second fight against Ballade happens in Dr. Wily's castle, and he even makes an appearance in a cutscene. Although there's not much one could really call a "plot," the few cutscenes are a nice addition that one wouldn't necessarily expect from a Game Boy game.
Another addition to this game is the appearance of a collectible letter in each stage. They are not difficult to get; the game wants you to get them all, and in fact you have to get all of them from the second set of bosses to enter Dr. Wily's castle. If you get all the letters from the first set, you gain a robotic bird companion named Beat. When deployed, he automatically attacks any enemies on screen. This is powerful, but Beat runs out of energy quickly if overused. Beat joins Rush, Mega Man's beloved robo-dog, who as in the previous game can be used for big jumps (Rush Coil) and flying over gaps (Rush Jet). I love Rush! The only thing that keeps Mega Man 2 from being absolutely superior to 3 is that the latter introduces Rush.
Dr. Wily's castle has three stages. They are harder than the earlier stages, especially one part where Mega Man must destroy torpedos that block his way without accidentally destroying the ones he needs to stand on. The Power Stone attack comes in handy here, as it can hit things below Mega Man, which the Mega Buster can't. At the end of the third stage, Mega Man faces Dr. Wily, who has three progressively more difficult forms. I enjoyed these fights. In the first two, Dr. Wily is inside a gigantic mech. For the first, you have to hit an orb suspended overhead (which is easiest to do with the aforementioned Power Stone because of its circular trajectory). The final form requires the Ballade Cracker, because it can be shot directly upward, where Dr. Wily floats in his iconic flying saucer.
I haven't said much about the gameplay because it's the same as in the previous games. The series is great because of its many weapons and because the platforming is so good. The first Mega Man was amazing compared to came before it (even though it had some unfun segments that required too much timing finesse). By the time Mega Man IV came out, Capcom had overcome any minor shortcomings. All twelve levels in this game are fun and well designed. There are jumps, moving platforms, and all the usual platforming fare. There are even a few branching paths, which pleasantly surprised me! Nothing wild or super innovative, but well crafted. I will offer one small gripe: screen layouts repeat themselves a little too often, a result, I assume, of the limited storage space in Game Boy cartridges. Even when levels repeat, the enemies change, altering the challenge. You could argue that it was an efficient way for them to cram more game into the tiny cartridge.
Speaking of enemies, Mega Man encounters a few of the cute and iconic, hard-hat wearing Metalls. The rest of the enemies are robots taken from the NES games. I've never learned any of their names because there are a lot of them and each game uses new ones. They often take the form of animals and are mostly generic. There's a humanoid called Crystal Joe that forms a crystal it shoots. The crystal moves slowly and is easy to jump over. None of the enemies is difficult, and Mega Man usually only faces one at a time. The game happily avoids spawning them in unfair locations the way the first Mega Man game sometimes did (taking a page out of 80s arcade games). They will respawn if you back up and cause the screen to scroll back. This can be used to farm energy refills.
One of the best things about this game is how faithfully it reproduces the NES sprites and music. They look identical to the originals, meaning the sprites, including Mega Man himself, appear rather large on the small screen. This is good because it means you can see everything clearly (unlike in, say, the first Super Mario Land, which used tiny sprites). It would be nice if the game were in color, but the monochrome is fine. The music is great; as far as I know, it's the same music from the NES game. Basically, this is a Mega Man game on the Game Boy without any drop in quality beyond what was necessitated by the small, monochrome screen. I enjoyed it and would recommend it to any fan of the Game Boy or Mega Man.
"Alternate routes, optional pickups, a store system, completely redesigned levels and the meatiest Wily experience yet in the handheld series make this an unfairly overlooked outing for the Blue Bomber."
— Philip J. Reed, Nintendo Life, 9/10