It's time to go after Dr. Cossack, the man who has brought you so much trouble. Note that the password you got after defeating the last Robot Master is the last password in the game. Yes, this means you have to beat the fortresses in a single sitting. If you didn't get the Balloon and Wire, go back to their respective hiding places and get them, then write the password down.
Another warning - unlike other Mega Man games, in Mega Man IV, there are no continue points in the pre-boss doors. Lose against a fortress boss, and you're going back halfway through the stage. Keep this in mind during boss fights. It's often more convenient to use an Energy Tank than play through half the stage again.
This is another level that is a mix of an all-out challenge from enemies and obstacles, and difficult jumps/platform stuff/ladders/etc. Rush Coil and Rush Jet will be your best friends in several sections of the level. Use them wisely, and don't be afraid to use Master Weapons, though you'll want Ring Boomerang for the boss. Remember, you can kill yourself on later fortress stages and continue to regain your lost weapon energy.
The boss is a flying drill machine. This thing is massive and it drills holes in the ground - the kind that falling in will kill you. There is a green jewel on the front of the drill machine, so unload on that weak spot with your Ring Boomerang. The boss moves slow and isn't much of a threat, so this should be an easy victory.