OP, cheating is cheating and believe me, I've felt the irreversible pain it does to you. Having been cheated on in the past, I completely understand what you're feeling, and I agree, direct confrontation isn't the way to handle this.
Like others have said, try to sort out the lease and whatnot and be ready. Get bags packed, find a place to stay, the whole nine yards. When it's all set in stone, you need to be honest with her in a way she wasn't with you.
"It's over. I unlocked the iPad, this is what I saw, nothing you say or do is changing this, the damage is done." Expect a dramatic reaction, of course. I remember my ex crying on the floor, saying it wasn't what it seemed like, it was all "a joke between [them]" (it very clearly wasn't), the whole "I wasn't getting the attention I needed" argument. This sort of stuff will mess with your heart and head. You loved this girl for four years, that love doesn't just die. It's going to sting and you're going to think, somewhere in the back of your mind, "hey, maybe I can fix this." It's not fixable.
Couples have survived cheating before, but for most of us, it's a scar that never leaves, even in future relationships.
You need to make your getaway plan, stick to your guns, vocalize the pain she brought you and let her know what she's done, then get the hell out of dodge. Then comes the healing process. Spending time out with friends, indulging in your hobbies, whatever it is. Clear her out of your life, delete her social media connections to you, block out any potential access she has to you and try to heal.
It's going to be a rough road but in time it will get better. If you need anything, PM me. I'm here to talk.
