Well he's not quite revealed, but considering there's a photo cover of the War Doctor, I'm going to guess that he's the fourth Doctor in the story. That makes the most sense, otherwise I don't think they'd give him a cover. It's pretty cool that writer Paul Cornell is using the current series companions. I kind of worried about that since it would be easy not to, but this actually builds on the work they're doing in the comics which is important as it sets the comics a part from the TV show.
August 12 sees the launch of Titan Comics' Doctor Who summer event - Doctor Who: Four Doctors!
This spectacular 5-part weekly event will be penned by Paul Cornell and illustrated by Neil Edwards, and stars the Tenth, Eleventh and Twelfth Doctors and their comics companions! The first issue hits comic stores on Wednesday August 12, supported by the global Doctor Who Comics Day on Saturday August 15! Each of the five issues comes with three covers to collect: an art cover, a photo cover and a companions art cover!