Look man... if you KNOW she likes you, play straight:
"Look, it's clear that you really like me. It's stupid to make things complicated, specially between you and me, we have known each other for 4 years and we can read each other out. Let's stop pretending and start enjoying."
Something on those lines. Also try to make her clear that this is her only chance to go out with you for real. You don't have to tell her that directly, but insinuate it with your attitude. You must look like "I'm the one brave enough to say what we both think and you won't have this chance again as if you say no you'll be the one who will be sorry later."
And that will be it, if she say no. If she say no, you don't have anything to be sorry about, you did your best, you told her directly, she said no. It's her fault, not yours. Move along, next target. It's very possible than in a couple of days she'll return to you.