So I've had the iphone 4 for less than a year but today the lock button stopped working... Is it still under warranty? Will Verizon give me the same phone or a different model? Is it free if it's still under warranty? Thanks for your answers and your time!
If you purchased it new and it is less than 1 year old, then it is still under warranty.
Take it to an Apple Store. They will examine it to determine if it stopped working due to a manufacturer's defect.
If it is covered, then they will give you a refurbished new phone of the same model. That will be free if it is covered under warranty.