Very high difficulty level. Same old tired storyline.
After making the 3rd installment of the Mega Man series a cakewalk, Capcom really cranked it back up in Mega Man 4. They also teased a new storyline by introducing a new boss of sorts, Dr. Cossack, who opposed Dr. Light & Megaman this time. A nice plotline twist, but ultimately a teaser as when you reach the end, you'll find it's none other than old Dr. Wily.
While the graphics & sound are their usual high standards again here, some nice changes were made to gameplay, namely the addition of the "Mega Buster". This weapon allows Megaman to charge up a more powerful shot instead of single weak shots, which adds a little more depth to your basic strategy. The other thing you'll notice, especially when you get to Wily's castle, is the difficulty has really been cranked up here. This isn't the cakewalk that 3 was-anyone who expects that will be frustrated beyond belief here, but those looking for a challenge wlil be right at home.
Although a "true" different bad guy other than Wily would have been nicer here, some credit is due for trying to mix up the storyline a little bit and keep you guessing. The high difficulty alone warrants playing this one, as the replay value is still as high as ever. Just prepare to work for your rewards.