Are you placing your new camera in the same place as this camera? I'm not sure exactly what your asking as if this is a replacement I would stay with a 4mm so you get the walkway/door/window and front corner. However our placement could present a problem as you are catching the backside of people as they approach.
As for Dome vs. Turret. Just keep in mind if you do a dome it will have to be totally out of the elements mounted under an eve where ran can not hit it. Reason being is rain on the dome will totally block your view and after the rain the water spots will distort your image and cause issues with the IR. So if your placement will be in the elements then you will really want to consider a turret.
Look at HIK 2mp or 4mp cams, stay away from 3mp as the image sensor has the most noise in it at night of the cams in there current lineup.