HINT: The other three are already married.

Guess which #Supernatural star just got engaged! http://t.co/GPlpNDwUMGpic.twitter.com/EJwmFpsY6g

— E! Online (@eonline) August 21, 2015

source

It's Mark Sheppard. He's marrying a 31 year old Supernatural fan that he probably met through conventions...(DON'T QUOTE ME ON THAT, I'M NOT A DETECTIVE.)