iphone 4s factory reset

[iMore Question]

I loaded ios 8 on a iphone 4s, and is running like crap. Anyone know if a reset of the phone, which there is a button for in settings, will make phone like new, outta the box, with a different operating system? Guy from at&t seemed to think so. That would be the only way I would feel comfortable, trying. Not about to figure out how to download some operating system, and then "hope" it'll work. Thx for any help. DM