First, it will be helpful to learn more about how Facebook works, and educate yourself on its rather robust privacy controls. Having four separate accounts is far more of a hassle than it's worth, and I'm not sure exactly what you are trying to accomplish. What are you trying to gain with separate accounts?

Facebook does not, and cannot, scan your computer for anything. Your web browser doesn't allow any website access to the filesystem. Like it or not, you have put information out there that Facebook is using to draw conclusions about people you might know.

If you insist on using four separate accounts, your best bet is to always access Facebook using your browser's Incognito or Private Browsing mode. This will delete any cookies when you close the browser window, and so nothing will be able to use your Facebook login for anything else. You will need to log in to Facebook each time you access it.

A far, far, better solution would be to consolidate everything into one account. You can set yourself as an admin for your business's Facebook page and choose whether to access and post to your business's page as your business or as yourself.

When you sign up for coupons, you are always prompted at the level of access to your profile you give the app. Frankly, if a coupon app asks for detailed information on me, it's not worth the 50 cents off.

As for a Facebook account for "private stuff"...Facebook is a social media platform. You are better off putting private stuff in a Dropbox account or something. Of course, you can always set the privacy control on anything in Facebook to yourself only.