I think there are two perspectives to consider here:
The photo-shoot and the post-processing is your gift to the bride and groom.
Simply putting these images onto the DVD media, doesn't take 4 hours.
If part of your gift to the couple was the full-rez images and all the rights pertaining to them, it might be ethically questionable whether these images are yours to sell to this guest at the party.
On the other hand, if you retain the rights, the post processing of the images is a gift to the couple and the question is whether it's "fair" to ask a guest for payment for this effort?
So, before even contemplating what is a fair price for your efforts/time/talent, the question about what exactly you are selling to this guest should perhaps be cleart first?