I'd go ahead and go with one of the "A in the class" letters, if I were you. You already have three other letters that can speak to your diligence/attitude/personality/etc, so if all they need is someone to attest to your academic capabilities, I think you'll be fine going with someone who doesn't know you too well. Do anything you can think of to make it easier for your writer to put together something fairly personal and specific to your strengths. Offer to send them your personal statement, or give them a few personal stories that didn't make it into your essays that provide some insight to your best qualities.
I'm surprised your academic advisor declined, you'd think writing letters for students would be part of the job description.
I'm sorry this happened, I can only imagine how stressful it must be to find out now! Good luck!