I paid for a commission to be done by Clay Mann at Boston Comic Con last month. He never showed up on the following day which was the last day of the Con so I never got the sketch. I'm still working on getting everything taken care of. I've accepted that I'll probably just take the loss in the end though.
My experience with Jason Momoa (Conan/Game of Thrones) was great. He was friendly and there were no problems. But I did hear from others during the convention that he was rude and that he was a touch/germ freak (he shook my hand no problem). I was standing outside and witnessed a moment where he blew somebody off but I can kind of understand it as you don't want to be swamped by people in an uncontrolled environment out on the street.
4 days is a long time. If they didn't have time to take on the commission then they should be aware of that. I would be extremely upset if I was you. It is common to have them mail you a commission from what I know though. If you don't mind me asking who is the artist? Most artists have a facebook page or deviant art page to help get in contact with them. I would also recommend contacting the people at Fan Expo Toronto and see if they have any information to help you get in contact with the artist. I had to contact the BostonCon people to get Mann's managers contact information.