Mega Man 3 (NES, 1990) - Snake Man
The third Mega Man game was a bit of a change-up from Mega Man 2. For example, it introduced a new move for the Blue Bomber known as the Mega Slide, allowing our hero to dash quickly through narrow passages or to evade attacks in a pinch. After its introduction, the Mega Slide was a mainstay in the classic series up until Mega Man 9 and 10, which removed this ability from the protagonist and gave it to a secondary character, Protoman. Speaking of Protoman (or Break Man), he also features quite prominently in Mega Man 3, appearing to challenge Mega Man every so often in specific stages and seemingly working with Dr. Wily. Another addition to the series is Rush, Mega Man's trusty robotic dog who functions similarly to a Swiss Army Knife. He can transform to help the player bypass certain parts of a stage more easily, or to allow them access to valuable secret items. Mega Man 2 had a few utility items that worked similarly, but from this game onward, Rush became the primary method for getting around stages a little bit easier.
Mega Man 3 was also quite a bit longer than the previous entries. After taking down the standard eight robot masters, Mega Man must tackle four extra stages in which he'll encounter large boss robots that possess the mannerisms of the masters from Mega Man 2...so basically, you have to take down two sets of robot masters in Mega Man 3, making for a quite lengthy and challenging experience! After that, you still have Dr. Wily's new and improved Skull Castle to deal with! Luckily, as you blast your way through these stages, a set of sweet tunes is also blasting away at your tympanic membranes. The musical style of Mega Man 3 differs quite wildly from that of the previous entries. Where Mega Man 1 and 2 generally just have straight up rocking tunes with loose association to their stages, Mega Man 3 has a much more atmospheric soundtrack, giving each stage a very unique audio signature. For example, Gemini Man's jewel/ice inspired stage has a very jewelly (?) sound to it. You'd have to hear the tracks in conjunction with the stages to really understand, but it's pretty cool stuff. My favourite track is that found in Snake Man's stage. Although it doesn't follow with the 'atmospheric' theme as much as the other stages, I find it very catchy and suiting to the level's somewhat sewer-like appearance.
Mega Man 4 (NES, 1991) - Bright Man
The fourth Mega Man game is my favourite of the NES entries. It's strange, because many fans cite MM4 as their least favourite one, but for some reason I've always liked it. Like its predecessor, Mega Man 4 introduces a new mechanic to the gameplay: the Mega Buster. Mega Man's arm cannon, previously called the Plasma Buster, has been upgraded and now has the ability to charge up and deliver a very strong attack that can deliver heavy damage to one enemy, or plow through a line of weaker ones. This ability is perhaps one of the reasons for people's dislike of MM4. Since this was the game that introduced the charged shot, it's also an ability that hasn't been balanced correctly. It's overly powerful, and can turn enemies that would normally be challenging into mere jokes. Bosses are also relatively easy to kill, even with the use of only the Mega Buster. Despite this small caveat, however, Mega Man 4 is still a joy to play through and offers polished, refined level design and a wicked soundtrack to go along with it.
Musical compositions for Mega Man 4 sort of fall into a halfway point between Mega Man 2 and 3. Many of them are atmospheric and well-suited to the stage design, but also have a more general rock sound to them, such as the tracklist of Mega Man 2. For example, Pharaoh Man's stage has a heavy, pulsing sound to it while also retaining an Egyptian sort of feel to it, and Toad Man's rainy, storm drain-based stage has a somewhat echoing sound to it that fits the bill perfectly. My choice of tune from Mega Man 4's soundtrack is that of Bright Man's stage. With a "bright" and electric sound to it, alongside a very catchy overall composition that always reminded me of Indiana Jones for some reason (which likely makes no sense whatsoever), I feel like it's definitely a standout track in the game. Embrace it, and just let it penetrate your skull, man!
...Skull Man is also in th...you see it was a...oh, nevermind.
Mega Man 5 and 6 are next, so keep an eye out for a new entry shortly!