Wily Machine 4

Wily Machine 4 (ワイリーマシン4号) is Dr. Wily's fourth Wily Machine, a skull-shaped flying machine that shoots energy balls. It appears as the penultimate boss of the Wily Castle from Mega Man 4. The Wily Machine 4 also has a brief appearance in the opening scene of Mega Man 8.

Strategy
The best way to attack the first form is with Ring Boomerang. Get close to the skull and shoot while dodging the energy orbs. This shouldn't be hard. Once defeated, the skull will vanish and Dr. Wily will appear.

The best way to attack the second form is with Drill Bomb or use the Mega Buster if the Drill Bomb has no energy left. Time it so you detonate the bomb and the explosion hits the blue gem just below Wily. Do this enough times to defeat the Wily Machine. But Mega Man still have to face the final Wily boss in the final stage, which is the beginning of the Wily Capsule bit that will continue to appear for the rest of the series. For this, use charged Pharaoh Shot, since it can be aimed, and you can even hit Wily with the charged up fireball without launching it, saving energy.

Another lesser known way of attacking the second form of the Wily Machine 4 is to walk underneath the machine and use the Wire adapter to hit the vulnerable gem. If the player is lucky, the wire will hit the gem once on the way up, and again on the way down. The second benefit is that if Mega Man is far enough underneath the machine, the machine's energy spheres can't reach Mega Man. Normally, the player will be able to take out about half of the machines energy bar before the Wire adapter runs out of energy. Then, use the Drill Bomb or Rush Coil to finish it off.

Rockman Complete Works data
ドクロじょうのフロントカバーを破壊するとビームのいりょくがアップする. ビームはよけにくいので弱点武器でさっさとたおそう Translation:

Mega Man (cartoon)
In Ruby-Spears' Mega Man animated series, Dr. Wily's main transport is a machine named "Skullker", which appears to be based on the Wily Machine 4. The Skullker has several features such as retractable arms, a tractor beam, ejectable seats, the Wily Capsule and the ability to attack with laser beams. In the episode Incredible Shrinking Mega Man, Wily equipped a shrink ray on the Skullker, but Mega Man turned the ray in the Skullker's direction when it was ready to fire and made it shrink, forcing Dr. Wily and Proto Man to abandon the vehicle. A weakness of the Skullker is that when it has water near or it gets wet, it doesn't work properly.

Other appearances
The Wily Machine 4 appeared in Rockman 4Koma Dai Koushin.

Trivia

 * This is the first Wily Machine to be graphically rendered so as to be seen in a roughly 3/4 profile. The first and second Wily Machines are shown in full-on side profile, and the third faces forward towards the screen; each of these three were drawn in a flat perspective without any depth. In the case of WM4, the angled view implies depth, which allows for a more three-dimension persepective in spite of the two-dimensional graphics.
 * After defeating both forms, you can actually "kill" Wily by using Rain Flush when he is going out of the screen. However, you must turn the power off or reset your NES, as you are left in space after "killing" Wily. However, in the Legacy Collection, you can use the rewind feature to rewind your Rain Flush shot.