Mega Man 4 (USA) ROM

After the third defeat and supposed death of Dr. Wily, the Earth comes under threat from a mysterious scientist named Dr. Cossack and his eight Robot Masters. Fearing the worst, Dr. Light sends Mega Man to save the world once again. Mega Man 4 carries on the same action and platforming gameplay as the first three Mega Man games in which the player completes a series of stages in any order and adds the weapon of each stage's boss to Mega Man's arsenal. One notable added feature is the "Mega Buster", an upgraded arm cannon that lets the player charge a regular shot into a much more powerful blast.
Despite Mega Man 4 receiving positive critical reviews during its early 1990s release, the game is widely regarded today to represent a downward shift in the overall quality of the series, particularly for its immediate sequels. The Mega Buster itself has been considered an overused mechanic by several sources.
 

Mega Man 4 can be considered a step forward for the franchise in quality over its predecessors. Levels are longer and more complex. Sub-bosses are no longer after-thoughts but rather complete events. The music is tighter and fuller overall. The sound effects are crisper. The graphics are more detailed. The 8 robot masters move with more fluidity. The castle bosses are complete battles. Unlike fights like the dragon in Mega Man 2 or the replicas in Mega Man 3 (which had an awful castle) which you could just smash the fire button at and beat in 5 seconds ... these final fights are really cool. Some true classic stages can be found here as well. Dustman is a primary example. Bright man and Ring man are also excellent and easily outclass the level design of say, Magnet Man or Metal Man. Mega Man 4 and 5 are easily the best games in the NES series, with Mega Man 6 not far behind. 

This followup to Mega Man 3 may not have been as good, but it still has the classic Mega Man game play we all love. I'd still recommend to at least try it, because you can't go wrong with good old 8 bit Mega Man! However, this was the first game to introduce the "two fortresses" concept, since of course Wily is behind Cossack's actions.