I guess you could process much more information out of that image with the same data, than you have here. Of course I don't have your setup so can't say for sure, but normally I'd expect more data in 4 min shots.
I took this last year, comprising of 2 minute exposures:
http://content.rogergroom.com/cms/000896.jpg
sure... different camera, but I'd think your DSI would still give you something near this for 4 min vs my 2min exposures.
13 shots at 4 mins is reasonable to have a good crack at a target like this in my opinion.