You'd need to install an app on each of the phones, kill the battery life of those phones, and you'd mainly be able to monitor their pockets. And if they're tech-savvy, they'll figure it out pretty quickly. To do it well, you'll have to pay for a real service, as well. And make sure that all your teenage children are under 18 or you're breaking multiple laws.
The least expensive solution would be "establish trust." If you feel the need to monitor *four* teenagers to this extent, something is likely really wrong with the relationship.