Mega Man IV is a game for the Game Boy. Mega Man 4 on NES is not the same game. Its game is the fourth of the five Mega Man games for the Game Boy. From a series perspective from the Game Boy, it is the last one that has robot masters from NES games.
Following another defeat from the earlier game, Wily is still up to no good. He steals the robots from the convention with the same goal as before. Knowing this, Mega Man must stop Dr. Wily from taking over the world once again.
There is the stage choice, which is the tradition of Mega Man games. There are four robot masters that will initially be present, and they are the NES game Mega Man 4. Stage select is not the same as the earlier games. Completing the stages will result in going to the mid-stage. After defeating the mid-level boss, the next four bosses will be from Mega Man 5 on NES.
For robots, there is one more that has continued to be the tradition of the Game Boy Mega Man games. Ballade will be the boss. Only once defeating all the robot masters in the game, you will be able to eventually fight him. He is part of the Dr. Wily levels. Once defeated, his weapon will be acquired.
There is the stage that is most known from the NES Mega Man games. To elaborate further, it is the stage in the Dr. Wily levels that has you fighting all the robot masters again. Defeating them again is going to be important before finishing the game. At the end of the game, there is the final boss. Dr. Wily is the final boss and once defeated, you have completed the game.
Mega Man IV is a good game to play. To find this game, it is likely going to be done online through Amazon or eBay. Playing it will not be difficult. To elaborate, it can be played through handhelds like the original Game Boy or the Game Boy Advance.