The title is probably confusing but I share a PS4 with my roommate. I want to buy PSN for online multiplayer and such but she doesn't want it connected to her account at all. So is there anyway that one account can be disconnected from the other account that has PSN because I already know that if one account has PSN all get certain benefits (online multiplayer etc). So is there anyway to do that?
Assuming yours and your roommate's accounts are two entirely separate "master accounts" and one isn't a sub-account of the other, it's absolutely possible.
By "buy PSN" I assume you mean PS+. PS+ is specific to individual PSN accounts when it comes to personal info, billing, etc. You don't have to do anything to set it that way, it's simply how it works by design. If you purchase it for yourself, she wouldn't be able to link it to her own account even if she wanted to. She'd have to buy her own.
Case in point: My wife and I both have system logins on my PS4 and both have our own PSN profiles associated with them. My account has PS+, hers does not.