This is a tough one, I understand your conflict. In one hand it is simply a bad idea; there are so many ways this can go badly for you... your bf could find out, things could get out of hand when you meet this guy and you might cheat in some fashion regardless of intention. The list goes on.
But in the other hand if you don't pursue it, you will always wonder what if... and that can eat at a person after time.
Here is my advice: you have been with your current bf for 4.5 years and are still happy and in love... being both of those things after that much time is actually not as common as you thing. In life it is actually the exception, not the rule. It is really not risking that over a 'what if' regardless how long you have known him through the internet (and thats certainly not the same as being with someone in real life). Find peace with what you have, count your blessings, and be happy with what you have. You are young, there very well be a time in the future for the two of you to meet that wont be bound by any guilt... trust me you dont want that!