To be honest I think what you're feeling now is completely just a reaction to what you read. As you said, you thought she'd never have feelings for you. Discovering that she does has shook you. As a result, you're questioning everything you thought you knew.
The point is: you were happy right up until this discovery. You had a partner who you love and loved you back and you'd put all these feelings behind you. That wouldn't have been possible if you hadn't moved on. You have moved on. If you hadn't, then you couldn't have found happiness with someone else.
She was a huge part of your life, it's completely and utterly natural that finding this out would stun you. But it's just your past feelings causing this. Four years ago, you were an expert at pining for her, so at the first sign she's available it's very easy to adopt the habit again. You're practiced at it. It's old hat to you. You probably wouldn't form an attraction like this to anyone new, because you know you should keep your thoughts loyal to your boyfriend. But being attracted to her is too easy by now to resist. Not without extra effort.
Honestly, I think you should hold on to what you have with your boyfriend. You have a loyal, happy relationship with him that is tired and tested and (above all) you don't want to leave him. So don't. Nothing whatsoever has changed since you looked at her laptop. You still moved on from her and you still love your boyfriend. A new possibility has just been opened up and you're naturally tempted. But it hasn't changed anything about your situation or what you achieved.