Well, the 4-series Gran Coupe will use the same wheelbase as the 3 Series sedan and the 4 Series Coupe, so in a lot of ways it is very similar to the 3 Series sedan. The only noticeable differences at the moment are the shape of the roof and the 2 extra doors at the back.
The 4-Series Gran Coupe is supposed to be 4,640mm in length and 1,825mm in width and 1,370mm in height. This make is only slightly taller than the 2-door coupe version.
Engine should be the same, performance will lag slightly because oft he extra weight of the doors, and lastly, the price will be higher.