Jailbreak apps

Hello All, Please forgive my ignorance, but can you purchase and use a jailbreak app on a non-jailbroken I-Phone 4s? I am looking for an application that kills all apps running in the background at once and I have noted several applications that do this via a jailbreak app, but I have been unsuccessful at finding a non-jailbreak application for this issue. Of course it would have to be VO compatible as well. I look forward to any advice/recommendations anyone can give. Thank you. -Andrew-