I'll go first:

Mr. Fan-Tastic:

His power is that he can turn into a gigantic fan of any size and blow enormous wind currents from a simple ceiling fan speed to devestating hurricanes or even just blow wind at nearly 200 times the speed of sound. He can also turn parts of his body into fans or rotors so he can fly and move around freely. He isn't too intelligent, but he's certainly smarter than the average human. Maybe around a 170 IQ.

Invincible Woman:

She's just invincible. Like, completely, utterly indestructible. Not even wolverine claws can cut her skin or hurt her organs. She has no super strength, no other powers, she's just impossible to damage and kill. She has absolutely no other powers besides that so she's kinda useless in a larger scale.

Torch Human:

He has light based powers, and can project any and all forms of radiation from his body at any intensity, from every spectrum of light, visible or invisible. He's arguably the most useful. Unfortunately he can't fly or do any of that, he's literal just a walking torch person.

Nothing:

If Torch Human was light, this one is complete darkness. Nothing is literally just a sentient void, has the appearance of a small, completely and utterly black sphere, but can expand and consume things around it and eat it out of existence. Nothing can grow to any size it wants, as long as there is edible matter/energy around it to feast upon and grow in scale.

How do they defeat Galactus

Mr. Fan-Tastic can turn into a fan and propel Invincible Woman at Galactus at the speed of light, essentially using her as a human sized bullet. Hopefully this should just punch a hole through Galactus, and if it doesn't, the others will try their hand.

Torch Human will fire a giant beam of light at Nothing to eat, consume and grow, until Nothing is a void as large as Galactus. Then Nothing will attempt to consume Galactus, and if necessary, Torch Human will keep feeding Nothing infinite energy and light until Nothing eventually grows far too big for Galactus to handle and then Nothing eats Galactus and reduces him to... nothing.