Rain Flush

Rain Flush (レインフラッシュ) is Toad Man's special weapon. This weapon creates an acid rain storm that affects all things on the screen (provided they are vulnerable to it), including shielded enemies like Mets and Pakatto 24, and can hit several times before disappearing. In Mega Man IV, it can extinguish the fire in Napalm Man's stage for a short time, and solidify the quicksand in Pharaoh Man's stage.

Damage Data Chart
Known damage values in units for Mega Man 4.

Capacity
Rain Flush uses 4 weapon energy units in Mega Man 4 and 1 unit in Mega Man IV. With a full weapon energy gauge, it can be used 7 times in Mega Man 4 and 19 times in Mega Man IV.

Bosses weak against Rain Flush

 * Bright Man: Mega Man 4 and Mega Man IV
 * Charge Man: Mega Man IV
 * Wily Machine: Mega Man for the Sega Game Gear

Other media
Mega Man uses the Rain Flush in episode 10 of Ruby-Spears' Mega Man animated series. Interestingly, both Toad Man and Mega Man use it in the form it takes as a useable weapon- firing a pod straight into the air, which then explodes and produces a cloud that releases the Acid Rain on targets below 

Trivia

 * The Rain Flush can be used as a substitute against Pharaoh Man if one has run out of Flash Stopper, though you'll have to contend with Pharaoh Man's speed.
 * If Rain Flush is used on Wily as he escapes the first battle in Mega Man 4, he will be destroyed, and as a result Mega Man will be stuck in the stage, forcing the player to reset.
 * The Rain Flush (followed by the Centaur Flash, Astro Crush, and Tornado Blow) is the first weapon in the Mega Man series to damage all enemies on screen regardless of defense.