I agree with GoneA BUT for a different reason. I do not think that taking it to the next level is always a bad thing in friendships..it really depends on the people involved, what their expectations are (if any) and how open minded BOTH of them are.
My agreement to leave it alone is based on the fact you havent jacked together in over 4 years............it would seem that he has moved on from a need to do that or he does not wish to pursue that anymore. What his gf looks like doesnt matter either....(does she have a great cock like yours- um no).

If the two of you were together more, hanging out etc and the opportunity arose, you can always ask him if he had ever considered it. I think most men, especially str8 guys, love the idea that you wouldnt want to do it with anyone else but him, its a common male ego thing ...
For now Id let it be, let it be a great fantasy- but if you ever start getign together again like before...approach him about it...or begin to go down on him and pause to see if he is ok with it....if he doesnt stop you then he wanted you to also- if he does stop you apologize and explain your feelings and that you just wanted to know what it was like one time with him...im sure it would go ok either way.

Good luck...........